Sorry about the delay in replying.
Yup so if I disengage the free-wheeling hubs at the front (so it's set to 4x2 mode), and then lock the center diff so I'm driving just the back wheels, there's a large clunk when taking up drive. I've done a bit more investigating and the rear propshaft is really short (44cm) and the front is quite long (78cm) so I'm assuming that when the engine/gearbox transplant was done, they didn't do the advised thing of move the engine forward to accommodate for the longer gearbox & transferbox. I've seen a few on forums that did it this way, but the rear propshaft is almost parallel to the ground, so the gearbox must be sitting really quite low (brand new GB springs on it, so it's not that). Might be able to counteract that with some extended shackles on the back.
The rear propshaft is almost fully extended and is out of phase, so it could be they fitted at series 1 rear (45cm extended) instead of making up a new one. I think once I get this replaced, the clunk should go away, but if anyone who's done the same transplant could point me towards some helpful info (or recommend somewhere to make up a custom length propshaft) it'll be really helpful.
Cheers