Pull the Craigslist ad if the block in the car is original. Not worth risking a numbers-matching combo (heads and block) just to avoid using a set of adapters.

A stock '78 400 would have some pretty low compression, depending on the heads (6x-4 vs. 6x-8). One way to kill 2 birds with one stone is do a stroker kit. Not only will the added cubes make more power, but will raise the effective compression ratio with those same heads.

This 6x-4 heads should measure 95cc or less, while the 6x-8 will measure at least 100cc. On a stock '48 400 that equated to a 8.1 and 7.6 compression ration respectively. Depending on the stoker kit, you can raise that by a full point or more. Gee, right in the ballpark where you wanted to be... 2 birds, baby!