Pistons size 3.5, 3.9 or 4.2?

Hi I was wondering is anyone could help got a rover v8 engine took the sump off and spotted these numbers on the bottom of the pistons please could you tell me what engine size they are if can be done thanks
Been told it's a 3.5 block with 4.2 internals but didn't think it could be done thanks
9400
R2601

I believe it is possible with some relieving of the block casting to clear the 4.2 crankshaft. You wouldn't end up with 4.2 displacement, around 3.95 on the standard bore.
number may be weight stamp so balanced sets are used. Try turning by hand and measure stroke.

OK thank you for your help