Q1 - no idea, I'd ignore it for practical purposes
Q2 - I'd go for the studs and sleeve nuts as per the alloy head VH engines - 3/8x16 is too coarse for the 'standard' head bolts used by the 350s, it might be the same as the studs for the VH. However, the holes in the head might only be 3/8" which would be too small for the standard VH sleeve nuts, or there again they might have already been enlarged in which case the decision's probably been made already!
Q4 - looks like a standard (probably NOS) older 350 piston, so probably uses standard rings (Drags seem to have them) - are there any numbers on it anywhere?
Re. Pete's remarks about oil pumps, personally I'd go with whichever pump you've got and see how it goes.
'55 Huntmaster, '56 VH, ' 51 VH, '80 R100RT, '00 Sprint ST (but all those Ariel parts can only make one running bike...)