1995 Gt 325k+ Miles Runs Great Rebuild?

An HCI upgrade is for a healthy engine. A325k mi engine isn't healthy. It may run OK but with that many miles on the recip assy, I doubt that there is 100 pounds cold cranking compression.
Anytime you add power to anything, you make it work harder. When stuff has to work harder, tired parts fail. More power typically results in more heat, and the weakest link is gonna show itself shortly thereafter.

The cheapest most often recommended "upgrade" is to find a used engine out of an Explorer, and swap them. But if you truly have a 4 k budget, you should be able to rebuild your foundation w/ new stuff, and have a fresh engine instead.

Here's the rub....

You'd be better off with the JY swap on the engine, cause you're gonna need the remaining 3500.00 for the "rest of the story"

Not only does the engine have 325k on it, so does the poor ol' T5, and 8.8. You put fresh power to either of those, and you're gonna have gear oil on you at some point in your near future. It's a wonder the transmission hasn't just died of natural causes already. A bearing/seal in the rear is just waiting on you to add 300 ft'lb of torque back into the equation, and I'd be surprised if the suspension bushings aren't completely gone too. Rubber bushings don't last forever regardless of how well the car was kept,..just plan on that, and you wont have to spend endless hours replacing ancillary parts that gave up the ghost the first time somebody sidestepped the clutch.

Well, you popped my ego with a measly 250k.

Like previously mentioned, other areas should be updated too. And, a rebuild with the hci.