The 425 and 455 Oldsmobile blocks are identical all the way down to the factory boresize. The crankshafts are interchangable.
The blocks are not "identical". Can't recall if all, or only early 425s had a different lifter bore angle. Not a major difference but one that should be considered if expecting the most out of a cam choice.
The crankshafts are
NOT INTERCHANGEABLE. There is a .275" difference in stroke so they obviously wont "interchange". Perhaps you should have said that either crank will fit either block(although the 425 may require clearancing at bottom of cylinders for a 455 crank/rod combo), but displacement will be determined by the crank chosen, and that the appropriate piston/rod combination will have to be used. The rear flange of early(1965?) 425s has a different bolt pattern. The 455 PTO will not fit this different pattern. I think Glenwood still lists the 425 item.
The 425 Olds all came with a beautiful forged crankshaft. If you have not yet seen one of these cranks, you will love looking at it. I think you should use the 425 crank, absolutely. By the way, they have huge rod journals and you can offset stroke these Olds 425 crankshafts and have a nice forged stroker crank. Aftermarket piston/rod combos will need to be figured out by you, then stroke the crank accordingly. By the way, I'd recommend using fully-grooved bearings in your Olds boat motor so as to establish continuous oil feed to the rod bearings.
Great idea if one is building a motor from scratch, but it is a pricey proposition as it will require a non-stock combination of rods and pistons. For someone just trying to put a 455 back together as a 455, the 425 crank isn't going to work without being offset ground to the 455 stroke. This will still require the use of non-stock pistons/rods. If just using the 425 crank as is, it would still require the use of 425 rods/pistons.
Its too bad that the Olds strokers were never as common as the Ford or Chevys. I always wanted to build a bigger Olds based on an offset ground 425 forged crank or even an offset ground nodular iron crank(had one in my 455).
