RE: Looking for scale body? Where?
A lot of the fuselage size application relate to the scale dimensions, relative to the length of the tail booms - 30's being shorter then 50's. So I would assume a thirty size would be better for you then a 50 size.
That being said, another consideration is that your Venture 30 is a belt drive, so you will not be able to modify the length of your boom. Consequently you may have to shorten the scale one on the fuselage or let some of your boom and tail gearbox stick out the back.
If memory serves me correctly, the Venture 30 has a pretty deep mainframe. That may limit you to the type of fuselage you can use, so with some, you may have to go to a larger 50 and shorten the boom.
Many times, in such a situation, you may find it more expedient, and a better bet financialy, to remove the radio and power system from your existing heli, sell the airframe, and use that money to buy one of their complete kits that include their scale mechanics.