If you swapped in the 273, you might want to hunt down a pair of 67 of later heads because of the intake issue. Another thing to consider is the earlier 273s had a smaller crankshaft register than the later ones. Something else to consider as mentioned earlier is get a 5.2 or 5.9 and build everything on the V8 K member while you are driving the slant around.
Early A’s use the same K member for v8 and slant. He just needs the motor mount brackets. If he goes aftermarket/Schumacher v8 mounts he won’t need the drop center link, as they raise the engine enough to clear the link.
The v8 throttle bracket and the exhaust manifolds are all he really needs from OEM, pick up a 318 magnum from the salvage yard and find a small block 904 and he is on his way.
