muncie and hurst shifter downshifting troubles

Sounds like a problem with the synchro if you have everything adjusted. It may up shift ok cause the sychros aren't needed much on up shifting. Have you tried double clutching to see what happens?

What the synchronizers do is they make the two gears in question slow/speed up so they are turning at the same speed and then you can shift from one gear to the other without clashing teeth. On each synchronizer assembly there are brass cone clutches that grab the gears, slow them down, and hold them steady for shifting. I find it unlikely that ALL the synchronizers are worn or broken at the same time and only on the downshifting event. What is more likely is that the clutch is not disengaging completely and this keeps the mainshaft turning, making it difficult or even impossible for the synchronizers to work. Very heavy lube oil can cause this too. There is a "spin down" time that the factory specifies to allow the gears to synchronize and heavy oil can cause the shaft/gears to spin a long time and make shifting difficult. This would also affect reverse gear, which is straight-cut and not synchronized at all. Shifting would be noisy and harsh. Have you tried bang-shifting? By that I mean shifting without using the clutch. For each gears change there is a moment when the engine rpm and gears speed is perfect and you can shift out of one gear and into another simply by adjusting engine rpm to match each gear. An example would be at 2000 rpm slipping out of first, dropping to 1000 rpm, then slipping into second. If you can do this then it proves that there is nothing wrong with the gears themselves and the other things mentioned are probably at fault. Anyway, that's how it works. Since you said you overheated the clutch a bit, I'm inclined to believe some clutch facing material may have melted and lumped up or even warped the disc a bit, and is preventing complete disengagement. When you upshift you probably instinctively take your foot off the gas and drop the rpm, making it easy to shift. You probably don't do the opposite on downshifting so try it and see. If this is the case, adjusting for a bigger clutch disengagement gap should fix it. Worst case would be needing a new disc.

The lube is fine, 90wt was what it used when new. But- " It was like I had to adjust it every trip for smooth reverse engagement." That is a sure indication that the clutch is not disengaging completely. Is there an inspection cover on the bottom of the bell housing? Can you insert a feeler gauge to see what the clearance is at the flywheel/pressure plate/disc when the clutch is disengaged? What kind of fork pushrod are you using? Does it have a locking nut so it doesn't vibrate loose? I prefer the BB Chevy style with the bent end and one part screws into the other. Maybe the fork stud is not allowing enough adjustment? There used to be three lengths available, for the three different pressure plate designs, flat finger, bent finger, and long. They do make an adjustable one. My very first manual trans swap ran into all these problems and more, so don't feel bad.

Sounds like you used top shelf parts all the way. What pressure plate? Another cheap trick is to remove the drive shaft and pop an old drive shaft sleeve in the tailhousing. Then with engine off, trans in gear and clutch pedal depressed, see if you can turn the output shaft by hand. A completely dissengaged clutch should allow that.

Same as TH-400, yes?