You answered your own question. The block and the crank don't match. You need to either get a different block or different crank.
Do both of the crankshaft pilot holes match your transmission? That might make the decision for you. Otherwise, the steel crank is the stronger for bearings. If you don't have much invested in block prep, then go find another block to match the crank. Or vice versa if you've already got premium pistions and overbore going on the block.
No big loss, either way, you can use the leftovers to build your 'Next' motor.
(I've currently got 3 in various stages)