Ultra bell question for overdrive 727 and 904 front pump differences

Quick question, Looking at ultrabell's to put an overdrive behind my 440.

Looking at them, I see that ones for 904's and 727's have a different part number. Is the front pump pattern all that is different between the two or is the bell housing size different overall?

I have found one for a 904 locally for considerably cheaper than one for a 727. I want to bolt them to a 518 or equivalent and put it behind the 440 for my truck.
Could I possibly modify it to work?

Can anyone help me out? I really want to put an OD behind it, but I don't want to shed out $400+ right now.

Thank you!