Pedal box help.

[CaptainCrash1971]

While in the garage rummaging through the boxes of stuff that came with my chassis I found some odd pieces of steel. After getting my nose stuck into the book, I realised I had a complete pedal box kit, probably from 3ge. This will hopefully save me a few quid, but when I found the donor car's master cylinder it is far to big for the hole in the pedal box back plate. The question is, do I fettle the hole and make it bigger to take the master cylinder, or just buy a new master cylinder?
Is there any difference in the different size cylinders?
Cheers
Alan