Hello, Will and welcome to the fray.
Proceed with caution on the Miata seat mod. I tried and did NOT like them. I'm 6'2", about 195lb. (34" inseam) and the Miata seats just didn't do it for me. They're larger, wider and sit higher than the stock seats (which are dinky and really couldn't hug the floor pans any closer). I found the seated position simply too high, also the wider Miata seat also inevitably takes away some of your right (accelerator) foot/leg room: this alone made me remove the Miata seat.
First, the slightly higher seated position with the Miata seats significantly limits visibility through the windscreen: you must practically break your neck to see stoplights! This may sound picky, but around town (where I do a fair bit of driving) it is a significant issue. No matter what you do the Miata seat is at least approximately 1.5" taller than the stock seat. Not much, but this is a critical amount of headroom in the GT6 for anyone taller than about 5'11"-ish. However, with the Miata seat my head did not touch the top/headliner, I still had a bit of headroom.
Also, normal seated position (with stock seats) in a GT6 requires folks with longer legs to squish the right leg up against the gearbox tunnel. Sadly, the Miata seat make this worse, creating a very cramped position for the foot/ankle/knee on the accelerator pedal. Even with stock GT6 seats this cramped right leg/foot position is the most uncomfortable aspect of the GT6/Spit cockpit. Other than this, I fit nicely in a GT6 and find the cockpit pretty darn comfy. Getting in and out is a bit of a chore, but if you have anything near normal flexibility, it's not a problem.
I tried all the Miata seat installation tricks including cutting down/trimming seat foam, and cutting out about an inch of the rear seat rails and welding them back together (in a futile attempt to get a tiny bit more leg room). All for naught, the seats were simply too big for me.
Will, at your height and weight you may find the Miata seats quite uncomfortable for the above reasons. Smaller folks have no problem with them, and some love the Miata seats. Not moi.
My tuppence worth: keep the car interior original. Recondition your stock seats. Do a thorough job of restoring them with new materials. They are comfortable enough for a bloke of your size/weight.
Finally, the Miata seats just don't look right in a GT6, IMHO. A Triumph GT6 is not a ricer, don't turn it into one.
I hope this seat diatribe helps.
