Wglcome to the club. Sorry I'm a bit late to the party but maybe I can help having owned one of my GTOs for 10 years. Btw, it is currently the most powerful, quickest and fastest in Europe.
As you have a 91 TT it is most likely sporting the 2 bolt main block which has the cast crank as opposed to what is dubbed as the forged Gen 2 crank. You can tell as the earlier engines have a CAS on the timing belt side. The only reason you may have the 4 bolt main is if someone has previously swapped the engine.
The bad news is only minor in that 450 BHP is probably your real max to safely tune.
The good news is that 450 BHP is a fairly well trodden path. Depending how much your budget is will determine which path to follow. Personally, your best and safest route is to go stand alone ecu (about 1500 fitted and mapped for a VIPEC), if not you can go with piggy backs but whilst this works, you are always trying to fool the ecu into thinking it is doing something different than it really is, so you always have to compensate here and there. For piggy backs you will need to be able to control boost, fuel and timing. Stay away from manual boost controllers.
Probably the best turbos to get you to 450 BHP will be either 13Ts or 15Gs. The former come as a kit as they require slight modification of certain bits to fit (very minor though), the latter simply bolt on but will be a little dearer. 13Ts I believe are around £800 a pair.
Fuelling: any of the following pumps. Walbro hot wired, Walbro 400 or 450, Supra turbo pump. Injectors either RC550 or a set of 450s from USA for a DSM.
Fuel lines should be fine but change the stock loop that joins the 2 fuel rails for a higher flowing one.
Plugs, go 1 range colder.
You can achieve 450 BHP with the stock MAF so should be ok
Depending on the condition of your side mount inter coolers you might be ok. Options are bigger side mounts or a front mount inter cooler.
Stock rad and fans will be fine.
Adjustable fuel pressure regulator.
I recommend at some point changing the output shaft in the gearbox for either a Quadco or 300M.
Stock HT leads are good to well over 600 BHP
Sorry for the unstructured reply, it was just a quick brain dump.
André
Ps Curd, I didn't know you were on here!