"...The car currently has the original 7.5 posi rear..."
If you plan to keep that rear, I wouldn't go with any more motor than a low compression 400. I'd go with 6X, 5C, or '74 model #46 heads, with either a 481988 or XX 481988 block.
A 262 Voodoo cam would perk it up a little. Or, to save a few bucks, you can go with an 066 clone cam, and stock valve springs.
https://www.ebay.com/itm/New-Sealed-...FZuHy6&vxp=mtr
https://www.amazon.com/Melling-SPC-4.../dp/B000C4KLKI
https://www.ebay.com/p/Engine-Camsha...SPC-5/77094380
https://www.ebay.com/itm/New-Hydraul...xZODlz&vxp=mtr
The 066 cam was last used in '76 400 4-barrel engines.
http://www.wallaceracing.com/enginesearch4.htm
The Summit 2800 has more lift & duration. And the Summit 2801 is a notch bigger, but will definitely need at least some new stock replacement springs.
https://www.ebay.com/itm/Pontiac-V8-...lU0mJ9&vxp=mtr