If you could do ONE upgrade...

Tires. Toss those old BFG T/A's in the trash and put some decent modern compound tires on there.

Of course, you'd need new rims to do that. And then you'd need new suspension to take advantage of the improvement in handling you'd get with the improved tires. And new suspension of course isn't even one thing, it's a whole bunch of things.

What I WOULDN'T do is spend a bunch of money on horsepower without improving stopping and handling first. But that's why it's never just one thing.