76 -- I got some vibration isolaters from McMaster Carr to mount my brownie. I've though about using them as cab mounts too. They come in various ratings. They might be thicker though and you might have to modify a bracket or two.
I thought the 450 probably had once been a tandem axle, as single axle trucks usually had 2-speed axles not brownies. Hopefully that's a 6031A. Much friendlier on the road than the B model that had a deep under. The remaining two ratios of the B split the main box into effectively a 10-speed, not nearly enough gears for a little 71-series. With the A model you can split the main box three ways which gives you about 250 RPM between gears -- 24% under, direct, and 17% over as I recall.
It still amazes me that these old trucks hauled such loads with such little power. But then I remember when I started out driving on a B-model Mack over the Cascade mtns. with a 170hp diesel and a Mack Duplex 9-speed grossing 60,000. The line of cars behind me on the 10-mile Vantage grade stretched all the way back to the river. It took about 45 minutes. The last tanker I drove grossed 105,500 and would pull that grade at 30 MPH. It had a 600hp Cummins and 13 speeds and didn't need all of them.
