I use Mazda pistons
STD bore 77mm. But I guess you can go 40th oversize to get 78 easily.
They are dished.....but when installed on the A14 rod with A15 crank they would stick out the bore so the tops are machined down till you get this


Yes they are flat.
Pin hole on rod needed to be enlarged to 20mm for the Mazda gudgeon too.

Piston I used was out of a Mazda UC OHC series engine circa 1979-1981. 323 RWD and E series van.
ACL part # RY2699.
They run thin (1.5mm comp rings)

Hope this helps.

I'd like some forgies to suit...and will need some flycuts for valve clearance before the bigger cam goes in.