I swapped out the B1 wheelset for the (6'6) Princess Royal Wheelset as the loco had lost a good bit of pulling power with the smaller wheels. With Princess Royal wheels, this problem was solved. GSWR 4-4-0s had 6'7 wheels, whereas the Claud Hamiltons had 7ft drivers. 6'8 or 6'9 would be the likely choice if one felt like going for the larger GSWR machines, with 6'6 for the lower end compromise.
Regarding 4-4-0s to 4-4-2s I've recently seen a 3d conversion advertised of the Bulleid conversion of a Q1 to the proposed 0-6-2 tank variant that he had proposed before moving onto proto leader designs. They've designed the bunker and faux chassis + slot for the bogie to seemingly slot onto the back of the Hornby Q1 model (and glued into place).
https://mademe.co.uk/product/never-built-range-q1-tank-oo/