The irony is that many other MII owners are trying to get OD so they can get better gas mileage on the highway. With a drag race friendly 3.55 or lower rear gear, you can expect 4000 rpm at 70 mph.
My problem was probably due to shifter fork wear. Eventually it can get so bad that you get the gears jammed. Also it can get such that it won't hold in reverse. Poor downshifting was my cause...i.e. downshifting at too high of a speed, and getting the shifter in the wrong place. If you are upshifting in standard fashion, it will hold for a while. If you are dropping the clutch, granny shifting, or hurrying your shifts in any way, you can expect failure.
If you really want a reliable drag trans that makes repeatable shift times, grab a functional C4, add a shift kit, new seals, and new clutches. Probably less than $500 for the whole thing. Almost any C4 will do (small and medium Fords '68-'79), but you will need the MII bellhousing, flexplate, and TC to bolt it up without mods. Keep it in mind that the bellhousing, block plate, flexplate, starter, and torque converter all have to come as a package. If you end up with an earlier C4, bellhousing, flexplate, and TC, the mods are actually no biggie, just remember to get the set. A functional C4 can likely be had for $100. With a little work you can find one that someone has already modified that will last you a season.
The 3+OD was a transmission based on the RUG (Toploader) that used an internal shift rail instead of the earlier external shifter rods. And its a 3 speed plus 1 OD gear instead of a 4 speed. Its pretty much as tough as a RUG. It does require the standard RUG bell housing though.
Lots of MII/302 owners are looking for good 4-speed parts, so you are likely to be able to sell it all. You may even find someone with a C4 setup that will swap even with you.