What should you expect to pay for a complete 75 $400 core

It's high considering most of it is not useful if you're building a stroker. You can't get much over scrap for low-po mid 70s stuff anymore either so selling parts you don't need to recoup may not happen. I paid $200 for a '71 400 block a while back when low deck strokers gained popularity. I couldn't give away a complete steel crank 440 a few years ago.