Here are the alternatives as I see it, for a B20:

1) The M40 - regular 4 speed Volvo box, as fitted stock to the vast majority of B18 and B20 powered cars
2) The M41 - an M40 with an OD on the back making 4spd + OD. Fitted stock to a very small number of 122's, a decent number of 140's, and almost all 1800's. Easy to fit, mostly just needs a shorter front driveshaft section.
3) The M45/46/47 family - these were fitted (99.99% of them) to slanted OHC engines. They are the later model (and tougher) 4 spd, 4spd + OD, and 5 spd (respectively) transmissions that appeared in the 200,700, and 900 series cars. An issue arises here because although the bellhousing to engine bolt pattern matches the B20, it was designed to lay the engine over and keep the transmission upright. So the transmission lays over when bolted to an upright B20. You can either find an exceedingly rare B20/M45 bellhousing (used for a very short time on certain late year '75 240 cars, possibly US market only), cut-rotate-reweld a B21 bellhousing, or I've even seen an installation that left the transmission tilted, with an angled shift lever.
4) An Australian company (Dellow?) makes bellhousing (or just an adaptor?) that allows you to use a common Toyota 5 speed transmission.
5) John Parker (V-performance.com) makes an adaptor for a B20 bellhousing (and other parts) that allow you to bolt up the common (at least here in the US) T5 transmission (commonly used in Mustangs behind a V8, also in other applications). Tough cheap transmission.

As you progress down the list the amount of fabrication goes up.
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I'm JohnMc, and I approved this message.