What is involved in converting a TH400 to a 4L80E, and what are the hidden costs?

What the swap takes (Jeff Smith, several columns):

• Transmission: the 4L80E is essentially a TH400 with a 0.75:1 overdrive; the first three ratios are the same. It's longer and heavier. Jeff estimates about 4 in longer than a typical TH400, depending on the extension housing.
• Driveshaft: usually shorter. Measure at ride height, not with the car hanging on a lift.
• Crossmember: probably relocated or replaced.
• Floor: full-size trucks should fit under the stock floor, but early A-bodies (like a '65 Chevelle) need the tunnel cut out and replaced — real sheetmetal and welding work.
• Controller: the 4L80E is electronically controlled, so it needs a standalone controller (FAST, Holley, HGM, MSD, Painless, DIY Autotune and others). Wire its power and ground directly to the battery; Jeff had electronic noise problems when he didn't.
• TPS: carbureted engines need a throttle position sensor added to the carb.
• Converter: the stock converter is tight and very heavy. A performance lockup converter suits spirited driving, and for towing a bit more stall helps get moving.
• Cooler and temperature: a transmission cooler for towing; keep ATF below about 225°F.

Where the extra cost comes from: the controller and TPS, a better converter, a new driveshaft, crossmember work — and in tight early cars, transmission tunnel surgery. Jeff notes the price adds up when you count everything, but the lower cruise rpm and drivability are worth it long-term.

Worth comparing: in an early A-body Jeff suggests a 4L60E instead unless the engine makes more than about 700 hp — it fits the stock tunnel (he only dimpled the floor for the servo cover) and costs less overall.