R380 rebuild - sticking when in gear

[rincewind23]

Hi all,

Building a locost with a Rover v8 engine from a Disco. Got the engine running with a new microsquirt injection/ignition system, sounds lumpy but great! Now looking at the gearbox.
It's an R380 from eBay - obviously the bolt holes don't match the original bellhousing so I dismantled it and swapped in the input shaft from the original LT77, and the LT77 gear casing so that the box mates with the bellhousing. I managed to align everything (i think) correctly, so the spools are in the right place, and all the gears engage.

My problem is that without the gearcasing on, when I put it in gear on the bench, I can turn the input shaft and get the output to turn. When I fit the gearcasing, I can't get it to turn.

Is there anything obvious to check that I could have done wrong? It might not be obvious (ha!) but this is the first time I've dismantled a gearbox..

Cheers,
KEv.