455s are more expensive cores due to the misconception that "bigger is better". Not so. ALL the 3.25" main blocks (including 421 and 455 "HO", but not SD) are "weaker" than 389 and 400 blocks (3" main). Yes, that means the cranks aren't quite as strong. To put that "in perspective", remember, BBC and Hemi have a 2 3/4" main. The 3" main Pontiac is not only "bigger" in the mains, but also about 4" shorter. In short, Pontiac CAST cranks are nearly as strong and tough as Chevy and Dodge FORGED cranks. Vinny Meyeda goes 6.80s over 200 with a nodular Pontiac crank and "stock block" 406. That's about 1,800 HP from CAST parts. He shifts around 8,500, too, so get the idea that a Pontiac CAN'T rev out of your mind. Feed it and oil it, and it will rev as well as ANY American V8.
The 389 is the "forgotten *****". The short block is very nearly identical to 400 except the final bore size (4.062" vs. 4.120"). Same crankshaft, etc. Using an Eagle "stroker" crank with "custom" pistons makes a VERY strong 458 CID engine.
There are three basic motor mount "systems" from '59-'79. The "two-bolt" mount (pre-'70) is the most common. All the blocks from '70-mid '75 have five mounting holes, supporting both the two-bolt mount AND the three. Later blocks only support the three-bolt mount, and probably should be avoided. The 557 casting (last 3 digits) is the "weak" one. The 988 is much better ('75-up).
As heavy a car as the '50 is, the larger cubes would be desired. If you "lucked upon" a 421, THAT would have "cool factor" beyond any of the others. They're tough to find, though. A 389 block with the 4.25" stroke, the '67-later heads (have the "good" valve angle, where the earlier stuff is 19 degrees) and a '66 TriPower unit on top (all available "new" now), would be the ultimate Pontiac "street rod" arrangement.
NOTE: The "advantage" of a shallower valve angle is a better "approach" to the cylinder through the intake path, providing a straighter "shot" at the center of the cyinder (intake) and "rolls" the exhaust valve over to make a straighter "shot" at the exhaust port. The practice of "angle milling" small block heads stems from the desire to improve both of these issues. It's not as significant for the small block's exhaust, as that port is excellent. The Pontiac needs all the help it can get on the exhaust side.
TH400 is the minimum level of transmission that will "live" behind a high-torque Pontiac. TH200-4R has been known to "take it" if the internals are upgraded. We've seen a lot of trouble with both TH350 and TH700-R4, simply not up to the task.
You should consider getting Jim Hand's "How to Build Max-performance Pontiac V8s" published by SA Designs. The original work was done in '04. There's a "rewrite" available (not by Jim, himself) but none of the principles were contacted for that. More "tractor motor" crowd stuff, rather than embracing the present (stuck in 1979...). Though a bit dated today, it still has many good pieces of information and history of the Pontiac. The old HO Racing and Pete McCarthy books are okay, but WAY obsolete.
Feel free to add questions here, or if you have trouble finding things, PM me. I'll point you to the right people. Pontiacs show more "cool factor" than most other American cars, and a Pontiac-powered Pontiac street rod is among the COOLEST!