The A14 stroke, at 77mm, is 5mm less than the A15. That means that the A15 pistons rise 2.5mm higher [& fall the same amount lower] than the A14.
If you use A14 pistons in the A15 engine, the top of the piston, now rising 2.5mm higher than before, will protrude by this amount from your block. This is fixed by machining this amount from the crown. If you take 1mm from it, then you will still have some piston sticking up past the top of the block.

If you take 2.5mm from it, you had better have a real good look at the distance between the top of the piston, & the top ring. By now it's geting pretty thin & the strength here is becoming marginal, particularly if you are pushing noticeably more compression.
I think you will find that this land is only about 5mm thick in stock trim, & that's not a real lot to play with.

Think twice, then act once. [Measure it twice]