As much as I like the Pontiac, keep the Ford "all Ford". As said, the FE engine is an excellent replacement for the old Y-block, and still relatively cheap and available.
While it may have a large displacement (513 CID), it's still just a "Pontiac". There are no "big block" or "small block" Pontiacs. There are "large" and "small journal" Pontiacs, but it's kind of a misnomer, as the mains in the "small" Pontiac are bigger than those in both BBC and Hemi. Those of us "in the Pontiac world" disregard the 265/301, as it has no real potential and is rather weak.
The early 389 block is best "saved" for early restorations. Nothing "special" about a '62 389, except it won't "fit" directly to a TH400. It has larger bolts (7/16"), though the pattern is the same as the later ones. The "kicker" is the starter is bolted to the transmission (like a Ford) and not the block, like the later years. It can be an "adventure" to get it all to work in concert.
Save that 513 for something in the 3,000 lb. range, so you can make "believers" out of the "crater crowd" and their 572 Chevys...(:-