The Jap ones came with them from the factory so they were adapted to fit the Sierra platform. They were an SJ30.
LJs have a shorter gearbox with a remote linkage shift. So you'd need a custom length jackshaft, new mounting system (LJs actually have the gearbox mounts in the engine bay near the bellhousing) and you'l have to make the linkage system work. Or you could buy this incredibly rare genuine SJ30 gearbox, you'd have to make sure the bellhousing patterns are the same though, as there's 2 different LJ50 ones.
http://www.ebay.com.au/itm/SUZUKI-LJ50- ... 512&_uhb=1 A bonus of the LJ gearbox is you get heaps better ratios for offroad.
You could try and create an LJ-1l hybrid gearbox like I tried to but as you saw, it didn't work out for the combo of parts I had. It could work with a different combo though.
The engine mounts are narrower but that's pretty much the easiest thing ever to fix with the way the LJ50 engine mounts. There's also sorting out the oil injection, exhaust and throttle cables.
The lack of power is a real issue offroad. A standard LJ50 actually has lower gearing than your Sierra currently does with the 4.16 transfer and they only have 28.6" tyres. Look up some vids of Tanshi and 3cyl at Ripley, the Galsshouse mountains and tough tracks (only Tanshi went to tough tracks) and you'l see what I mean, they both have 6.5s too.
It will be awesome but it will be worse offroad, I'd keep the F10a. Staying 4T is probably 1/5th of the cost and it'l work so much better. If you do go ahead with it and you can actually find a good motor (they were rare 30 years ago, let alone now) I've got a ton of random bits if you need anything. There's a wrecker back home that has a gearbox in each bell housing pattern but for the price of shipping you'd just buy the one on eBay.
You'l get the best results chasing up leads locally and door knocking on farms.