If it's a stock shortblock and there's no V/P clearance now, there shouldn't be any with the new heads and same cam.

It never hurts to check, but don't let that worry stop you. You'll have about as much clearance as you have now. Think about it, the Source head is basically a copy of a 915, and the 906 is the same except for the open chamber. Valves are in the same location in space. You can usually run the stock rockers on the Source heads, too.
R.