I was watching an Arnold Schwarznegger movie (there all stupid, but mostly entertaining), he got into a Sunbeam by ripping the seat out first. Not a recommendation.
Funny thing about Arnold, he is certainly broad but really not that big, Mike Golic, former defensive line player in the NFL tells a funny story about how he was a fan and finally got a chance to meet him at a party, he said Arnold kept running away when he would try to talk to him, I think Arnold is 5'9" or 5'10" and at the time anyway, didn't want his picture snapped with anyone that would make him look (relatively) little.
On the subject of size and cars, interesting how different people fit differently, I am 6'0" and have plenty of leg room in my TR250, I have the seat set in about the middle of the track adjustment when driving, plenty of room to go back further. MGBs have a goodly amount of legroom too, and a little more elbow room, Austin Healeys are better than TRs on the elbow room too, but I would say worse on the leg and head room. Funny, I just read a contemporary road test of the TR250, and they said you would have trouble fitting if over 6'0".
I think in addition to people's body dimensions differing a bit, preferred driving position differs a lot, I know my brother's legs are shorter than mine by an inch or so, but when he gets in my cars to drive them he always pushes the seat back a notch. My first car was an Austin Healey Sprite, so I learned to drive with my knees bent and legs splayed out a little, and all other cars seem big by comparison, people who come from the SUV and pick-up world often think "our" cars are just too cramped, even though they physically fit in them.
Best way to find out of course is to have him sit in one, if a TR4 isn't handy the whole TR4, 4a, 5/250, 6 range of TRs all shared the same cockpit so have him sit in any of them for a trial fit. That being said, an overstuffed re-upholstered seat vs. an well worn, sagging original will also make a difference in how much room you have in the car. Common trick in the early Healeys, which have very little headroom, is to have the seat cushion foam shaved to fit better, my head hit the top with the roof up before I did it in mine.