The GT6 cockpit is relatively small, and some folks with poor bodily flexibility or leg/back problems may have difficulty with ingress/egress.
However, I'm 6'2" and 200lbs. and had absolutely no problems with comfort in any of my GT6s. And, my head never touched the ceiling (stock seats). In fact, I found <span style="text-decoration: underline">ample</span> of head room, with at least 2 inches, maybe more in stock GT6s (warning, Miata seats may significantly reduce headroom for taller folks).
If you're relatively healthy (flexible) and not taller than about 6'4" or so, you be as snug as a bug in a GT6. I think it's just fine for everything but very long/multiple day road trips.
Although the GT6 is a true GT car, it is not the most comfortable. But then, who buys a GT6 for <span style="font-style: italic">comfort</span>! :laugh: For example, an MGBGT offers the driver a noticably more comfy driving experience (monocoque construction = less rattles; it's a safer car in a crash; larger interior, etc.). But then, it's <span style="font-style: italic">not a Triumph </span>(body bolted to frame = rattles, shakes; more of a "driver's car", etc.).
There is <span style="text-decoration: underline">plenty</span> of aftermarket support for GT6s (especially MK IIIs, the most plentiful GT6), and you can find help and expertise in abundance in the web. Spitfires share most parts with the GT6, so parts, by and large, are readily available.
A word to the wise: nice examples are increasingly hard to find. Good luck trying to find a nicely-sorted MK II with no rust. IN fact, there's a nice-lookng MK I on EBay right now! And if you get one and ever wish to sell it, keep it as close to stock as you can with mods that are reversible (keep the old parts). Serious collectors simply will not pay top dollar for obviously modified examples.
Go for it: I don't think you'll regret it if you have the budget and skill to enjoy the car.