i got this one used off of ebay. exactly how does it mount up. it has two round thick plates... i know i have to drill through the cavation plate and mount the top two to the bolts on the top cap. but those plates on the bottom? do they sit on top of the cavatation or do i set them under to keep the shower from comming up?

Round plates on top of the cavitation plate. They look adjustable to set how deep they go.

how deep do i set them? bravo 1

My pickup only protrudes about <1" (say 3/4") any more and it might bend in the prop wash or catch on something.

Set the depth by just having the tips fully protude below the cavitation plate. Any lower and it can cause problems with the water getting to the prop. It is adjustable, so set it as shallow as you can with the opening fully exposed.
Then have someone come up next to you and check the water flow at speed. Or if you can talk your wife into laying on the sun deck and taking a look, that might work. I know mine wouldn't..
If you need to you can adjust it deeper.. Like everything in boating.. tune for results.

Hope this helps.

Mr.gadgets is right. Too deep and the prop can suck air and cavitate. You want it as shallow as possible and still work at speed.