Hi Mike
No real problem with you using a 750 bottom end providing 620 Heads, barrels and Pistons are used, But as mentioned by Phil oil-ways etc can differ,
There are 3 varieties of 750 Cases that I know of, the old carb'd cases the first of the 750s with the clutch slave on the left and the later ones which do away with a O-ring between the cases and the barrels
I still have my spare 620 for sale at £2200
http://www.ducatisportingclub.com/sh...d.php?t=176410 The one James is advertising is another good bike I know as it's one I sold last year