Thinking about installing through hull water pickups and was wondering.... instead of
installing block off plates on the drives.... I had thought about redirecting the hose coming from the
drive into the boat.... turning it up and directing it back out the transome above the drive. Thoughts?

What kind of drives? I do not see why that wouldn't work. Curious to why you would want to go with through hull pickups? For chillers?

Bravo 3s. The boat had trs drives and it still has the clam shells in the bottom so I was just going to
put those back in operation

Our Bravo boat is rigged similar as the water from the drives is redirected thru the extension boxes to the top of the drives. The water to the sea strainers is PU from thru the bottom pick-ups.

I just figured by doing so it that way the water still flows through the drive and could cool it at the same time.

It's been done before, but don't do anything to the drive until after you get your new setup straight.

There are pics of folks that have done it here on OSO, both routing in and back out of the boat, as well as drilling a tapping a fitting into the side of the drive, and routing water back to the top.

Personally, I think that using the drives to feed your intercoolers would be cooler.
Do you have generator and/or air conditioner? You might could also use the water pickups from the drives for either of these when running.

Or you could fill the thru bottom clam shell holes and use the drives. Those can't help the speed.

I always had a thought of 400-450HP and BIII's on my 40 if I ever turned it into coastal cruiser. I'll be curious of your results.

Hopefully you can do a search...Tinkerer made a block off (with holes so it can drain slowly) the exhaust passage in the drive , to hold water as long as possible above plane, to aid in cooling it.

I set mine up exactly as you are saying.

Water for the engine cooling comes from the bottom of the boat,
Water into drive comes in the boat and is directed out on top of he top cap through a stainless fitting.

Do you have any pictures vintage chromoly?