Lots of inaccurate info here except from a few. Let me correct the myths.
1) Don't bother with the restrictors, they don't work and waste of money
2) The only reason a 425 will "rev higher" than a 455 is because, apples to apples, with the same intake, cam, and heads, the 455 will run out of breath sooner because of the increased demand for air from the larger engine.
3) To make even 500hp from this combination you'll need very good heads, more cam, a good intake and at least headers and sizable carb. And if you're going to make any amount of power pitch the rods and pistons and buy new, modern ones.
4) The balancer AND timing tab are different. But you can use a later model timing tab and new balancer with no issues. In addition there are many offerings for universal bolt pattern flexplates as well.
5) And finally, getting cams for a 45* block isn't a problem, even a Hyd. roller. I actually prefer that bank angle as the lifter is in a more vertical position.
I'm the one that brought the Mahle piston for these to market. I offer cams and other pieces as well. I also have 428-434 c.i. stroker kits for Small Block Olds engines.
Thanks.