I'd say he didn't. It's not that hard. First of all, the gearbox in all Triumph roadsters before the TR7 comes out from the top. Even the GT6 gearbox comes out through the interior though the lack of a convertible roof limits one's working space.

You remove the carpet section over the gearbox, which requires removing the shift knob. You remove the passenger seat, takes less than 5 minutes. You remove the vertical dash support. You remove the 8 or 10 little brackets holding the tranny cover, then remove the tranny cover. You remove the clutch slave cylinder from the bell housing. You insert a floor jack under the oil pan (with sufficient padding to avoid crushing the pan), and lift the load off the rear engine mounts. You undo the rear engine mounts and the bolts to the drive shaft flange. You remove all the bell housing bolts. You raise the floor jack so that the rear of the gearbox clears the driveshaft but no so much that the bell housing hits the firewall. Then you yank backwards on the gearbox (wear gloves) and the whole thing moves backwards on the clutch input shaft. You then use sections of 2x4 cut to about 2ft long to leverage the gearbox out form under the dashboard. Once you get it over to the passenger floor pan you just lift it up and over the door sill.

Okay, I'll admit that's a lot of work.