Slayer, I might assume that you seek to make a GT6 perform something like a modern sports car, with power, handling upgrades, etc.
I suggest that you drive a nicely sorted GT6, or at least ride in one. A local British car club member would be delighted to take you for a ride, I'd bet. What you will find, if the car is mostly in stock condition, is an antique car that does not accelerate, handle or ride anything like a modern car. The stock engine puts out about 100 HP (but can be easily modified to about 140 or so). Zero to sixty times are anemic compared to even modern family sedan. Plus, the cockpit is cramped; there is practically no safety equipment (it's a death trap, really); it's very hot in warm weather, and handling is <span style="text-decoration: underline">awful</span> compared to any modern sports car.
From a modern automotive perspective, it's a lumbering, rattly car and it will cost you a small fortune to bring it up to modern standards. I've seen GT6s with Ford 302 power; Rover V/8s, etc., for example, and practically every engine in between. To do this you have to literally tear the car <span style="font-style: italic">completely</span> apart and start over. The frame must be reinforced; the suspension completely redesigned; braking; cooling, the entire drivetrain totally re-engineered and replaced with stronger components. At the end of the day, it's no longer a Triumph which is a <span style="font-style: italic">profound</span> tragedy because these are very rare cars.
Even if you have the budget to do the above, why bother? Why not get a modern car that doesn't need all the modification (and MONEY) to achieve your desired results?
Please let me suggest that if you're set on a GT6, get one and modify it using period-correct components. This can be done with a modest budget, in fact, the GT6 is one of the least expensive LBCs to restore. With about 140 HP, a GT6 is <span style="text-decoration: underline">loads</span> of fun to drive and that growly six sounds like no other engine, IMHO.
Keep it original: in the end, you'll have more fun, and the car's value will be far greater.
Thanks for posting, keep us informed.