Unlike the Ford or GM performance programs, the Chrysler performance programs were highly evolved and they did not rely on the hot rod magazines to distribute the information. From the early days of the old Direct Connection program, the information to put together particular parts combinations to achieve particular performance goals has been readily available in their series of Race Manuals that any Chrysler Corp dealer can order and which can also be purchased online from several dealership websites and maybe even on the Mopar Performance website.
The first race manuals were over 1000 pages and were compilations of the earlier Race or Tune-up Tips that dated back to the Sox & Martin days. Looking somewhat crude in their earlier forms, with typewritten pages, the latest versions have expanded to cover specific areas instead of one HUGE book. Each area was written by factory engineers and tested by same. Hence, you'll find information in there on all kinds of things that you'll not find anywhere else. Things like how to modify particular intake/carb combinations for better fuel distribution and power output by placing "popsicle sticks" dams in the intake manifold runners. Basically, if you follow their directions, it'll probably put you to about 85% of where you'll need to be to win drag races and have a strong running street car.
I'm not sure about the cam specs of the P-series cam mentioned, but if it's got a 110 degree centerline, you'll probably have to recurve the distributor to the faster curve to get the most out of it (by observation). A centerline more in the 114 degree range is basically where all stock cams are and will ensure greater manifold vacuum at idle, which typically might not be a real issue if the .050" duration is in the <230 degree range for a 360 inch motor. Valve lift is in the mix too.
Where the intake manifold vacuum at idle CAN be an issue is when you put the trans in gear and put the brake on, as if you're stopped at a red light or stop sign. In the earlier times, power circuit enrichment started at about 6" Hg, but later two-stage power valves that Holley started using in OEM applications in the '70s would do a slight enrichment at 10" Hg with full flow at 6" Hg. This let them use a main jet about 2 sizes leaner for cruise economy and better emissions during cruise. Carter carb metering rods started out with a two-step rod and then started using three-step rods with the AVS to do the same thing. In one respect, that "in gear" vacuum reading can be more important than the "in neutral" vacuum reading if the cam starts getting past the OEM HP specs.
The Mopar Performance Porting Template package is a pretty neat deal! If you read the instructions and cut the templates carefully AND pay attention to some details, you'll end up with a porting job that took many thousands of hours to work out (at factory expense). Basically, though, just port matching the gasket surfaces will be fine for a reasonably stock motor, similar to what the NHRA specs are too.
Unlike earlier times, intake port velocity and the quality of the air flow are prime considerations other than just sheer port size. Again, lots of good information in the race manuals. A cam's timing events can compensate for a poor port flow situation just as a well designed port might not need quite to much "cam" to get great power out of the engine. A side issue in this respect is the "stroke to bore" and especially the "rod length to stroke" ratio. The latter determines how much "hang time" the piston sees at the TDC and BDC portions of its travel and similarly how much of a yank it puts on the intake charge when the valve opens.
The Dr. Gas "X-Pipe" would be the ultimate crossover pipe situation as it would allow each pipe to share gas flow with the other pipe, effectively increasing the total flow of the exhaust system--or at least that's the theory. In a late model Lincoln website, they also said it made the 4.6L Modular V-8 have a higher pitched exhaust note at WOT and that TWO of them in the system made it sound like an Italian V-12 (or something to that effect). Some drag strip testing that was done in a Pontiac enthusiast magazine several years ago had mixed results, it seemed to me. It was a "secret weapon" of one NASCAR racer for a while, until he flipped his car and went airborne and the x-pipe was exposed to the world--that was before they went to the same-side exhaust pipe exit location, I believe. The basic flow of the system is more related to the sizing of the pipe, though, than whether it has a particular style of crossover pipe. I'd aim for something in the
2.5" size for any medium or larger displacement V-8. Chrysler's systems back then were usually better than anything GM and especially Ford did, with respect to muffler restriction, sound, and pipe sizing.
The whole idea of the crossover pipe is to quieten the system. Ford typically used them on every dual exhaust they built back in the 1960s. Chrysler only used them on a very few applications, as did GM. In a V-8 application that had no heat riser passage in the intake manifold, or one that was blocked off, they might be more critical in the sound of the vehicle's exhaust as the head/intake manifold crossover passage might serve the same purpose as the H-pipe in some cases.
Whether an H-pipe or an X-pipe, both might also serve as a budget driveshaft loop should the front U-joint break--at least until the driveshaft might beat through the exhaust system. If a racing sanctioning body indicates you need a loop, do a "real one".
In searching for a muffler shop to do a custom exhaust system, do some highly informed shopping! Some shops might have highly experienced installers and the same shop might have some of lesser abilities. Some also might consider their work "good" when it really isn't. Personally, I like exhaust systems that are OEM configured and use clamps with OEM hangers so that I'm not beholden to a muffler shop guy with a torch if a muffler might need replacement--just my orientation. There was a deal on Two Guys Garage earlier today on Shaffer Industries (or similar) that did mandrel bent exhaust systems in OEM configuration with larger than stock pipes. Looks interesting as few companies will have existing large diameter pipes for the larger chassis Chrysler products.
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