Hi Ben - the Celica 5 speed worked out very well for me. Back in the mid 80s it was almost impossible to get an M41 or a Volvo overdrive 'box without spending huge dollars. Mine may have been one of the first Dellow conversions; it used a modified Volvo bellhousing and an adapter plate. We used a part Volvo, part Toyota clutch, a modified Volvo clutch fork, and (IIRC) a Toyota throwout bearing. For the spigot, we kept a standard Volvo bearing and added a sintered brass bush to it. A lot of things were worked out on the fly.
The only body modification needed was to nip about half an inch of metal from the transmission tunnel so the gear lever didn't foul
However, things have progressed a long way since since then, for example here is a picture of Stuart Inwood's Celica conversion found on volvoadventures.com. It's a good example of a very nice installation.
Note in that picture how he's used the auto transmission mounting holes (further back along the chassis rails) for mounting the transmission crossmember. Directly below the gearbox sump plug, you can also see two of the mounting holes for the standard M40/M41 crossmember
The ratios in the Celica 'box were not very different to the M40/M41 ratios and the 5th gear made a huge difference in fuel consumption when highway cruising.
If you're planning a conversion, remember that you'll need a six-bolt flywheel to fit a B20 if the engine is original. They changed in 1974 to the 8-bolt flywheel for the B20 and carried that change through to the B21 family.
That car of yours is looking great! Girling 4-pot front caliper and piston kits used to be available everywhere; I hope that's still the case.