A 5 speed from a 50's GMC 400 series may bolt to existing bell housing. Usually they're direct drive but sometimes overdrive, they are pretty large, half again as big as the SM420. Clark is probably the brand to look for, some but not all have the GM bolt pattern. Don't think a Spicer bolts in place of the SM420.
You'd want to look up the ratios for what you find and see if the extra gear fills your gap. They will have a low first, like the SM420.
Otherwise you can consider the overdrive transmissions listed earlier and swap back to a lower axle gear.
