good luck finding an M47 that mates with a B20.

Best relief is an Overdrive fitted tranny.
M41---fitted to B20s---you do know that the 1975 240 was the last year for the B20. From 1976 on the engines were OHC---B21, B23, B230.

Fitted to the B20- M40 4spd and the M41 with OD

The M45 4spd and the M46 with Overdrive were fitted to the the B21...series.

The M47 for the B230 began in 1987 US model(1986 elsewhere)
The speedometers get a signal from the Rear...there is no cable or speedo gear in the M47.

Since you have a M45, the easiest would be to swap in an M46--the OD version of the M45. (Check the Driveshaft length with the OD trans)

I had a 75 with a M40 4spd----and yes it is a screamer at highway speed.
But remember in the US mid 70s to well into the 80s, the National Speed Limit was 55mph, so it really didn't matter. Radar detectors were big sellers back then.