Can anyone tell me if the A12 motor will bolt into my B10  { existing A10 motor} with a four speed box or are modifications necessary thanks David

If the dizzy is at the front like your A10 it 'should' bolt in
If its further back they run different mounts

I use the b10 flywheel with A12 clutch, A10 fly is factory lighter/lightened and less stress on any of the boxes you choose to use  I ruined an A10 box using the A15 flywheel after heavy hammering and when I switched to lightened fly on a worse for wear A10 box yet had no issues it kep going strong then went for a dogleg box 5 speed which I hated as prefer normal H pattern shifting.

I gather you got your 4 door sedan? any pics??

It is strange the A12 sits so high whilst the A14 and A15 are taller yet sit about the same in this department but one solution if using the oval port heads as most peeps would know is to use the vanette low mount manifold which goes down then up to the ports. This saves some room in this department but also Im still to finish a single sidedraft SU manifold which would keep things simple and look good.
The problem is I went for the 2 inch carb as I planned on a A15 big sleeved bore 1800cc engine but will need a head plate to make sure the bore and honing is spot on without shifting as this is critical when sacrificing the original bore walls that supported 75mm then removed to take 83mm bores. The pistons are shaved ca18de and would need custom one off head gasket.