Hi Kevin;

You most certainly have a H6 trans today, and as one of the earlier posts state you will not gain anything when it comes to the revs on highway. To do this, you will need to change either the rear axle or modernize with an electric overdrive (including quite some bodywork to fit in this kind of gearbox in a PV).

Volvo had an early 4 speed gearbox called M4 (and not M40 which is a later construction). The M4 is bolt-on (both B16 PV and B16 120's/Amazons were fitted with this as an option), but you will either have to shorten the driveshaft by a few inches or get a driveshaft from a PV fitted with a M4. My personal experience is that the M4 is not a very solid gearbox, I had several where the syncromeshes did not last very long. I am today driving a H6 which appears to be extremely solid compared to the M4.

The M40 is a far better gearbox, but I am almost certain that it is not directly bolt-on to the bellhousing of a B16 (or B4B if that's what you've got). The M40 was developed for the B18 which has a different bellhouse. BUT it is certainly possible to do the modifications.

Whatever you'll do, good luck!

Ola
-58 544 B16B
-88 245 B230K