if i have 16 in vac i should use a number 8 right? but they only have 8.5 so do i go to a 7.5?

I'd use the lower number one 7.5". Most Holleys have 6.5" power valves which should be fine.
The Quick Fuel Power valves are about 1/2 the price of the holley power valves, and they have many in the half inch of vacuum increments.

The QF units are a better price but they are also the four door (high flow) units.
Note: QF has better pricing on most of the Holley type carb parts.

If your vehicle is an auto, you should go at least 2" below the in-D vacuum reading for your pv selection. I know, pv's don't allow fuel at idle, but they might if someone has a higher than normal idle rpm. This is why the 2" below the in-D vacuum reading is a good start.

Yes you need your in gear vacuum number then 3 or more below that the less radical the cam the more you drop say it's 14 in gear a 8.5 will be fine!
I once had a 318 with a mild cam that I ran a two stage 10.5/8.5 worked good!