Hi Roger and All: The 300B achieved 355HP thru the use of a higher compression head. I assume it was just the regular one milled, but do not know for sure. The 355HP heads were assigned different part numbers, 1730369 and 70. This would be the case whether or not other changes were made besides just milling to achieve the 10 to 1 comp. ratio. The Chrysler letter to dealers, dated June 13, 1956 announcing the availability of these heads, does state they are not interchangeable with the 9.0 to 1 heads on the 1955 model "300s". Certainly the larger (3" dia.) exhaust system would also help with power increase, although for gross HP rating such as 355 was, most of the system would not be in place. Of course, these changes were mainly for the purpose of keeping Kiekhaefer (and others) competative on the racing circuit. I think he pretty much got what he asked for! Hope this answers some of your question. 300ly, Gil [Non-text portions of this message have been removed]