[Coley]

Ok....so $500 is your budget, so we have to work from that.
Now, from time to time someone is selling a good, used 350...they generally are a dime a dozen to find.
If that is an option (?), find one that is already (still) running and do a compression check, prior to purchasing...and make sure you give it a good overall inspection.
If it is already pulled, see if there is minimum warranty on its condition.
Again...there were billions of these things made and put into just about everything for 30 years....so a good running used one for $500 is not much of a stretch.
Long term, when you have more funds....you can consider a rebuild of the original or perhaps get a crate engine.

My two bits.
Coley