340 pistons in a 360

I've just pulled the sump off an older built but as new 360 I have here and the part numbers on the pistons are 2899479 - which are 340 pistons?

Was this common practice and guesstimates please on what sort of comp I'd be looking at with standard j heads.

Logic tells me the pistons would be too tall to work and would require a fair bit of machining but thats what is there in front of me.