While the 305 is running good, save up your money and buy an engine a few pieces at a time. Start with a rotating assembly, 383 stroker (one piece rear main seal)...all forged. Next, heads...Trickflow or AFR's. then on to a cam (small base circle), valve train, valve covers, gaskets etc, etc. The last thing I would get is the block (remember, 1 piece rear main seal). Get a used (seasoned) block and have it hot tanked, bored .030" over and clearanced for the crank. (the shop may want your rotating assembly for the machineing process) Have them assemble the short block at this time.
Once that is done, build up the rest of the engine and have it ready to install. Now, when you are ready for the engine swap you'll need the transmission rebuilt (if an auto)...a performance rebuild. If it is a 5 speed maks sure it is a "WC" (world class) T5, it will have WC on it in or near the casting numbers. It will also need a performance rebuild.
Once that is done and you have the car back its time to swap engines, this will take a couple of days if you also do the following and take your time.
If you want to keep TPI/FI (if it has it) you'll need to get a different intake and runners, Accel or Edelbrock intake and SLP or AS&M siameased runners, port the plenum for the runners and where the throttle body bolts on.
Another route would be to get a modified LT1 intake like john sells here
http://lt1intake.com/services.htm This has the benefit of shorter runners for higher RPM operation and less gaskets/pieces to mess with.
This sounds like a lot of money...and it is but you can do this over the course of a couple years while still driving your car.