Check out the Clifford Engineering web site. Clifford at one time had pistons maybe a kit to put a 292 crank in a 250. The block and cam has to be clearanced internally for the rods, for starters.
IF the exhaust pipe is too large you won't create enough back pressure for the headers to flow properly.
Do you Leo Santucci's "Power Manual"? You can find it on the left under "Club Stuff".
