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I haven't been here in a quite a while, though I have been on Hybridz since the late 90's. I did a small stint with Porsches. (Daily drove a 944 and put 140K in three years up here in the Great White North!) Went back to Z's with a very low mileage 280zxt. I got tired of worrying about putting on mileage on it, so I sold it and bought a 370z touring. I've been mildly modding it over the last year. No more crazy turbo swaps into S30 series for me. I did move again and got myself a large detached 2 car garage that I have been upgrading it over the last few years with insulation, heat, lighting, flooring, tv and stereo (obviously lol). No more working on a gravel driveway for me! I actually enjoy having friends come over and wrenching on their cars for fun! I may be doing milder stuff these days, but I still have my hand in custom fabrication and porting/polishing, and generally working on stuff that improves general VE of engines, weight savings and parasitic losses. Stuff I can do on my own without resorting to expensive "bolt on's"

4th gear doesn't use the countershaft, it's direct drive. That's a good clue. Sounds more like the countershaft bearing. Looks like you'll need to split the cases, so you might as well do the mainshaft/input shaft bearing also. It's not a super-finesse job. Just needs some ingenuity. It's fun. Part of Z ownership. Here's a good illustration of the parts you'll be looking at. I'm sure that there are "how-to" videos out there somewhere. You can get a factory service manual on the internet that shows how to take it apart. Good luck. https://www.zhome.com/ZCMnL/tech/240SX5spd/Transmission2.htm

"Grinding" is not very specific. Can you add details? Have you driven it? Is it the same in every gear? 71B's are known for destroying the adapter plate bearings. Did you find any metal on the drain plug magnet? If you've been driving it you might drain the fluid and see how it looks. Might also be worthwhile to lift the car and get underneath with the engine running to see if you can locate the noise. Replacing the main and countershaft bearings is not very difficult. You only have to split the cases and pull the bearings off of the ends of the shafts. The adapter plate bearings are more difficult.

The LV1 only came in vans as far as I know. The LV3 is more widely available but has displacement on demand (DOD) that needs to be removed. You can pull the balance shaft out and they work fine from what I've read. For some reason these are super pricey, which seems odd as there should be a bunch on the used market. They came in a number of trucks and small SUVs. The LV1 and LV3 are 100 pounds lighter than the equivalent V8. Scoggin Dicky has some dyno videos where they used a pulled LV3 with DOD delete and a new cam and it made 368 HP and had over 300 ft-lbs of torque at 2 or 2.5K.