Buick V6 225 or 231?

There should be a part number cast into the top of the block in the back, on the ledge where the trans bolts up.

Valve covers are 225, which means the heads should be as well. I have both motors, and the valve cover bolt pattern is ever-so-slightly different between the 225 and the oddfire 231. One could put 225 covers on an oddfire 231 - you'd just have to elongate a couple of the bole holes.

Similarly, you can put 225 heads on a 231, but they won't oil properly. 225's oil through the head and all the later Buick motors are push-rod oilers. You could use hollow pushrods, but you'd only get oil to the pushrod socket on the rocker, and nothing to the fulcrum or valve tip.

Blah, blah, blah. If you have 225 valve covers, the motor should be a 225.

I have HEI on mine as well. You can use an aftermarket intake, like the Offenhausers to avoid grinding your original if you'd like. be aware that the EGR crossovers in the intakes are different between the 225 and the 231, so you'd probably need to cut a soda can to make a block off plate for your gasket.

If you want to preserve the stock carb and looks, grab a junkyard oddfire 231 manifold. They look very similar, and the one that came stock on my 76 231 already had the bolt hole clearanced and a button-head screw in place. They also have the same intake port configuration, I believe. Again, you'd need to fab EGR block off plates for the intake gasket.