One thing you have to remember when swapping out a Slant is you have to swap EVERYTHING. Not much of anything from the Slant bolts up to a V-8.
There's a lot of little things you can do to make a Slant better, but if you want a tire smoking torque monster prepare to open your wallet.

The motor in my Valiant would be fairly easy to duplicate for not a whole lot of money, and a smaller cam would make it a decent street motor too.