Thinking this through, I wouldn’t blame the rear end first until the basics are proven. A stock early Spitfire rear is likely 4.11, which is short, but with a TH350 in 3rd gear, 55 mph should only be around 3,300–3,600 rpm depending on tire height and converter slip. Annoying, sure. An issue for a Chevy 350? No.

First thing I’d verify is whether the TH350 is actually making it into 3rd. If it’s stuck in 2nd, 55 mph becomes 5,000+ rpm, and suddenly the whole circus makes sense. Confirm GPS speed, actual tach RPM, tire diameter, real rear gear ratio, and what converter is in it.

To confirm the rear gear, don’t guess by year. Jack it up and count it. Put the trans in neutral, mark the driveshaft and a rear tire. If it’s an open diff, jack up one rear wheel and leave the other on the ground. Rotate the lifted tire two full turns while counting driveshaft rotations. A little over 4 turns is 4.11, just under 4 is 3.89, about 3.5 is 3.55, around 3.25 is 3.23/3.27, and about 3 is 3.08. If both rear wheels are up and both tires turn the same direction, rotate the tire one full turn and count the driveshaft directly. The open-diff trick is the key: one wheel up means two tire rotations, or the math gets drunk.

You can rough-check the converter with a quick foot-brake stall test. Warm it up, hold the brakes hard in Drive on flat ground, stab the throttle just long enough to see where RPM stops climbing, then get out of it immediately. We’re talking 2–3 seconds, not roasting the transmission like county-fair meat. A tight street converter may show around 1,500–2,000 rpm. If it’s more like 2,800–3,500+, a loose/high-stall converter is adding to the problem.

If it really is a TH350, 4.11s, tiny tires, and no overdrive, then yes, it’s geared like a squirrel in a coffee can. The clean fix is an overdrive with lockup, like a 700R4/4L60. With the 4.11s, that puts 55 mph around 2,350 rpm locked and 70 mph around 3,000 rpm locked. A better cruiser would be 3.42–3.55 gears with overdrive, but I’d prove it has 3rd gear before tearing into the rear end.

1964 Spitfire has a 4.11 rear end, and what you’re saying makes sense. Where did the motor and transmission come from, and what kind of stall speed is the converter?

To make this work you’re looking at 3.08 gears or numerically lower if you want to keep the TH350. It’s still going to rev high on the highway, but I’m assuming you aren’t going to be taking this for long highway cruises. 2.73s are the right fit, but it’s going to have a really lazy launch which isn’t the reason you put an sbc in a triumph.

Replacing the transmission with a 700r4 is the single part best answer, but the best all around solution is going to be a 700r4 and 3.55 or 3.42 gears.

You could also look to Gear Vendors who make an overdrive unit for the TH350, it would still be pretty spicy and pricey, while the stall speed of the converter is a big part of the story that’s missing.