First thing you need to figure out is if you care about it being original? If you do then you just answered your own question. If not then you have endless options depending on your budget. With that 400 you can make a nice engine but it's a smog era dog so you will need heads at a minimum. Also depends on how tired the 400 is it may need a lot of work.

Consider this as a starting point... you can go buy a complete rebuilt very nice 400 engine for between $6-8k and be done. They are not rare. Or you can go meet with an engine builder and go over all the options for your engines.