Thanks @RoverP480, I got excited and did what you said...tightened up the castleated nut on the back to try and seat the mainshaft better. I was avoiding this part until I had the transfer case out. Seems there are two part numbers for these tin-y "Oil thrower" things, depending on which transfer case you have...and I didn't want to smash them before I knew which one I needed. But I threw one of them on there and tightened the castleated nut(a true pain, even with the gears seized). Dang it, it's still seized. I took it all apart and I'll have a good look again this weekend.

Good thing is I got a 10 pack of those bellhousing-to-casing paper gaskets. Ruined 2 already.

Side question: Do you folks use gasket sealer on that gasket? Or smeared in grease and just put on? I've always used gasket sealer on American cars. But a lot of these LR paper gaskets go on dry or with just grease to hold them on while you tighten bolts elsewhere on the truck(swivel balls, hubs, etc). Gasket sealer sure complicates meshing up the pinion gear to the layshaft gear. Seems to get all over my hands while I'm reaching around in tight spaces! Also, it's a 1 shot thing, and I have to scrape it off between tries. I'm inclined to not use it...but I'd hate to leak gear oil there. When's the next time I'll have it all apart again after its installed in the truck, right?

I have the smallest of three distance pieces on the layshaft (0.125", I think).

I can't work out how reverse is supposed to work. It seemed jammed. Looks like it will fit either fully forward, or fully back...but won't clear 1st/2nd Synchro. I mean, the Synchro/layshaft won't turn...so maybe it would clear if the gear teeth meshed and it could slide by?? It's hard to see in there with the bellhousing on. I even got a small ear wax pen camera in there to see.

The worrying thought I have is maybe the brand new bellhousing is machined wrong...maybe slightly askew, and pushing the shafts over a bit? It wouldn't be the first time a brand new aftermarket LR part was machined wrong. Meshing up the pinion gear and layshaft is the usual pain...but even when I do get it lined up...the last 1/4" doesn't want to go. I chalked it up to having to press in the layshaft to the bearing. But I wonder if something is just going in crooked? I have to use the bellhousing-to-case bolts to drive it down the last 1/4".

Also, a couple times I offered up the mainshaft, I tilted it down a bit and everything just slid off the back. So maybe I don't have everything seated? Those 1st/2nd Synchro brass pieces just fall off like bracelets. It's like I have to keep all the back half together while driving it into the bearing. Gravity is not my friend. The best position to keep it all together is with the front side of the casing pointed down. That puts the mainshaft loose bits on top, held in place by gravity. But driving it in like that is near impossible. Maybe I'll use the castelated nut and an impact wrench to "draw it up", like RoverP480 said.

Oh well. I guess I'll try again next weekend.