TH350, TH400, or 4L60E
Three GM automatics still end up behind street small-blocks and big-blocks. They are not the same trans with different pans. Pick the one that matches the engine and the way you drive, then install it like the factory would have: cooler, controls actually set, converter matched to the cam.
TH350
Three-speed, 1969-on, the compact one. Vacuum modulator plus a kickdown — cable or electric, by year. Lighter case, shorter than a 400. Fits a Camaro tunnel without drama.
Fine behind a street small-block. A healthy 350 or a mild 400. Not the trans you put behind a four-digit big-block and then act surprised. Governor and modulator set part-throttle shift feel. People “fix” a soft 1–2 by pouring in additive. Adjust the modulator and check line pressure before you buy a shift kit.
TH400
Three-speed, 1964-on, the truck and muscle unit. No TV cable. Detent cable or electric kickdown. Vacuum modulator on most. Longer, heavier, more intermediate clutch capacity. This is the one you put behind a torque-heavy big-block or a blown street car that still has to drive home.
First gear is deeper than people remember. Gear the rear end for the highway or you drone. The extra length means a different crossmember location and a different driveshaft. The old 350 shaft will not “just reach.”
4L60E — and the 700R4 / 4L60 it came from
Four-speed overdrive. Fourth is about 0.70. That is the highway gear a three-speed never had.
700R4 / 4L60 uses a throttle-valve cable. Wrong TV adjustment burns the 3–4 clutch. That is the number-one homemade failure. The cable has to sit on the right carb or throttle-body stud, and the setup is a specific pull at wide-open — not “tight enough.”
4L60E is electronic. It needs a PCM or a stand-alone controller. You cannot drop it behind a carbureted 350 and expect it to shift because you put 12 volts on the case connector.
Lockup converter: if lockup never applies, you make heat. If it applies at the wrong time, it chugs. The 4L60E family is not a 400. A stock 4L60E behind a serious big-block is a fuse. 4L80E is the overdrive that belongs in that conversation.
Cooler, yoke, speedo
Any of these behind a muscle car needs a cooler. The radiator stack is not enough if you tow or sit in parade traffic. In from the trans, through the auxiliary cooler, back. Do not reverse it.
A 400 is longer than a 350. A 4L60E is a different length again. Measure the driveshaft. Speedometer driven gear, or a GPS sender if you deleted the cable.
Customization here is the choice, then the install. A $400 trans and a $40 converter is how you buy the job twice.