I may be wrong (I haven't been able to do the research yet), but I think the H42 is the tranny out of a 60-series, is that correct? When I do the diesel swap I'll be installing a NV4500 (low low first, high high 5th). The '74 (mine's early enough to still have a 3-speed) was the first year for a 4-speed, and I'm pretty sure there's one that will more or less bolt in with the proper bell housing. Shortly after I get funding for my company, I'm hoping to track down a bolt-in set up so I don't have to do a bunch of mods to the frame that will be redone in just a year or so. I really appreciate the offer though!
The 60-series distributors, I've been told, are the vacuum advance ones that will net me additional fuel savings. I ultimately intend to track down and install some 3.70 third members (mostly for the diesel) to go with the 33" tires which will counter the speedometer deviation of the tires and make it correct again while also lowering the gearing to a more appropriate range to take advantage of the diesel's extra torque. The 33" tires combined with the 3.70 R&P will be very similar to 35" tires with a 4.10 R&P. What engine (etc.) are you running that gets you 17mpg with the 25" tires?
Additionally, thinking back on the transmission piece, the 4 speed is almost a three speed with a lower first tacked below (second and third are shifted a little to even things out, but not too much), so it might be possible that the 1.5F in there currently would be able to make better use of the gearing as well...