Nice pictures 
MDTM, could you do me a strange favour? Measure (roughly) how long the 'fingers' on your engine stand are (or can spread at max spread)?
I bought a Sealey ES550 only to find that it's fingers don't go nearly wide enough to reach the bellhousing bolt holes on the 390 (it has a huge bellhousing, admittedly) and apparently the next size up Sealey stand is even smaller :-/ I'm struggling to find one that'll reach.
I will try
I have an ES350 (Clarke one but its exactly the same and from the same factory) and I had the opposite problem as I have no bell housing adaptor plate fitted at the moment so its a bit narrow!
Cheers

The length of a 'finger' and the width between the widest point of the mounting slots on the back plate would be perfect actually - then I can just compare that to mine

Failing that I suppose I'll just have to get some longer fingers made up and hope it doesn't all collapse under the weight

(Mind, that tractor engine must weigh at least what the 390 does? About 350Kg?)
I couldn't quite believe the size of the bellhousing on the 390 though - biggest flywheel I've
ever seen!