1994 Astro 4.3L Stock Engine...

Leaks oil like mad. (Every 80 or so miles, I need to add another 2-4 quarts...)

I suspect its the front main oil seal, but I haven't tore it apart yet. (The leak is originating much closer to the front than the oil filter.)

Any suggestions? Or should I start looking for a 350 Crate engine to swap in?

Don't look for a swap yet. Wash the suspect area with a good cleaner. Top up the oil. Run the engine and see exactly where it is leaking from. If it is the front crank seal, replace it with a new one. The crank pulley will have to come off to replace the seal. Give it a shot. Good Luck.

Also check the oil filter mount. I had one develop a crack and any time it was hot it would leak a quart a week. New part from a dealer is about $60.00