The poly has the engine mount ears in the same place as an LA.
Let me rephrase that the LA has the engine mount ears in the same location as the L poly Motors. Because the polys came first. And the LA's copied them. If you magnaflux a 318 poly you should find you can bore it to a 340 bore size. Throw a 4-in crank in there and you should have about a 416? As for putting La heads on a poly block it can be done you have to block those two big holes in the water Passage on the top of the block.
As for headers I have not tried it although I have a set of them here but I was told the fox body 302 headers can be adapted to the poly head. As I said I don't know if this is true.
