[Steve Barcak]

Shad,
Yes there are some differences. Yes, these blocks are far overlooked for performance useage. Remember, forged cranks can be repaired and stroked/destroked properly by welding, changing the stroke by as much as 5/8" offset grinding done with the casr units everybody is playing with usualy chages it .050". It is not difficult to make over 400 inches with a 370 (even without welding) and reap the virtues of the stronger block, forged crank and smaller mains. You will need to research main bearing availability, it may be a problem. As far as head interchange. Any Pontiac head will fit any Pontiac block with no exceptions. There are differences but, they will fit. Remember that early motors were stud oilers and have an oil galley that goes up to the head. This must be plugged when using late heads to prevent hemeraging (an 1/8" pipe plug does the job just fine). Side motor mounts didn't arrive until 1959 but, like you stated, that is not a problem with motorplates. Timing cover interchange is not a problem. The flywheel pattern will need to be researched and may need some modifiing as well as the oil pump mounting.
Most Pontiac guys avoid these early motors because of some fabrication involved to fit late parts but, it is worth looking into. Personally, I wish I would have started playing with them sooner.
Some of my projects are based on the '59 and early '60 blocks as I am using 3" mains with displacements from 389 to 577 inches (all are blown /injected) although I have been working on an alum Ram Air V version and soon a billet version.
Depending on how long you have been into the hobby you may or may or may not be familiar with the brilliant minds of Craig Hendrickson and Kern Osterstock who formed H-O racing around 1974. These guys built (among other things) an animal 326 inch '65 Tempest ( based on a '56 316) that used Ram Air IV heads, a custom tunnel ram and they buzzed it to around 9,000 rpm I believe.
These men, overlooked when discussing Pontiac performance, did more as at least as much to promote Pontiac performance and development in the 70's. Then, they disappeared around 1981 when Ken Crocie bought the company (as best I know). Ken occasionally posts here. Ken, If you read this, do you know what ever happened to these 2 men? These guys have developed the early motors ('55-'58) further than anyone I have heard of.
Cam/lifter/dist interchange is not a problem either. Their are some other differences too I'm sure but, nothing that some effort and imagination could solve. This is really looking into. You can find and build a stash of early blocks by just looking for them. They are not considered valuable and they are around if you keep looking.
Shown is an older picture of one of my '59 blocks clearly showing the lateral stregthening rib that all earl

[This message was edited by Steve Barcak on August 20, 2003 at 02:31 AM.]