how hard is it to switch a 1982 carb 305 to a 1990 tpi setup?

 

It's time-consuming, detailed work involving harness change-outs, dropping the fuel tank for pump installation, getting into a lot of hard-to-reach places, changing a whole bunch of parts.

But, it's not "hard", like installing a 6-point roll cage in the car. Get as much of the factory system off of the donor car as possible. Don't settle for an incomplete setup, as cost will skyrocket if you have to go buy new stuff for what's missing.

You should know that the '82 exhaust system is terrible. The '90 TPI single cat exhaust was the 2nd best one they made for 3rd gens (dual cat the only one that was better). That's important, because performance will suffer unless the exhaust is upgraded.