The first 'mod' you have to do is all the preventative maintenance. And you have to replace the parts that have timed out.
Vac hoses, check valves, plugs, wires, timing set, oil pickup, fuel pump (and hanger mods), hot wire kit, valve springs, VC gaskets, fluids, modern MAF and Translator, 60's and a matching chip, manual boost controller....
And do all the free stuff like cleaning out the intercooler, zip tie on the accelerator cable at the gas pedal, oil pump mods while the timing cover is off.....
THEN when you have a ''stock'' car (that's over 300hp at the wheels) you can start doing actual mods. If you build a house on a crappy foundation, it sinks into the swamp. Same thing with modding cars.
...or you can think about stuff like your 30+yo rubber brake lines, lower ball joints, wheel bearings, and a powermaster that's going to fail soon...