slant six to big block swap

If that's you budget, your best bet is a running big block drivetrain in an undesirable car/truck. BUT, in my opinion (and the opinion of most) you also need some suspension upgrades, rear end upgrades, bigger brakes with discs up front, bigger radiator, etc. Money can be saved by building your own mounts, etc. Expect to spend twice your budget and you might get by.

You can't build a motor for your budget...