Tony,
I'm sure you will get opinions that go both ways on this, but I for one do not recommend using the later crossmember for exactly the reasons you site.
While there may be some minimal gain in terms of weight distribution, it just doesn't seem like it would be worth the headache. Things get tight at the firewall, and sliding the transmission back makes clearance tighter in the tunnel. Kickdown linkage can be an issue, and you will have to lengthen the cooling lines. Dropping the tailshaft by using a 4-speed type mount and crossmember helps with the tunnel clearance but also changes the driveline angles.
I build transmission mounts for a living, and did some prototype mounts for this application. There was no good mount-only solution, everything we tried still resulted in unsatisfactory results for the customer. For this reason we do not offer a mount for this conversion and do not recommend it.
Is there a particular reason you want to use the later k-member?
Steve Charette
<arnsr@xxxxxxxxxxx> Sep 23 07:28PM -0500
ok if you use the 66-70 k frame in a 62-65 b body it moves the motor back so what do you do about the trans mount and headers being so close to the trans cross member
tony