We're just getting in to a 1952 ranch home we bought and flooring prep is underway. Bedrooms have 3/4" oak hardwood over 45* plank over joists and the rest of the original main floor has 45* plank over joists and plywood screwed down. The main living space had carpet throughout.

The heat/ac vents currently sit flush to tops of the old oak floors and sit on topof the plywood in main space. Because the main living space has plywood that's only 1/16" inch below the oak, we'll sand the 4-6' into the rooms to have a nice transition and not have to demo the oak and lay new plywood.

Question on best practice coming..

When installing the new 3/4 hardwood over the oak to the walls, the flanged vent openings will not accept the louvers. I think trimming the hardwood flush to the face of the installed vent covers will be the right way to go. What's best practice?

You will be able to remove floor vent covers to be able to clean anything that drops into the floor vents. Old louvered vent covers can be replaced with new metal or oak. We cut the flooring up to the opening with an eighth inch wider than the opening so new louvered vent covers can be installed and removed as necessary. They will have the flange a half inch wide that will cover the edge of the flooring. If you will be adding new oak over the old and this is an eighth inch lower, you can use tar paper the shim the low part up instead of sanding the eighth inch of the plywood. It takes a lot of time and energy to sand an eighth inch off of plywood. Check the area with 6' straight-edge to be sure that you want to go ahead.

These are the wall vents along the baseboards. The duct work is flanged flush to the front of the wall opening and with 3/4" new flooring I was wondering if the installer should cut each board butting up perfectly so the new boards nestle up to the vent in a straight line with the bottom of the vent sitting below floor level, or run the new planks to the vent opening and find shorter(height wise) vents to replace the originals.

The best solution is not to trap the vent cover in place. To help with this the flange closest to the floor may be cut. If possible raise the ducting so everything clears.
You would need to have some knowledge and basic metal work to do this but it can be done. Self stick aluminum tape can help with the solution.
It is going to be a lot easier to move the vent location before the flooring is installed.
If you decide to go ahead without moving the vent, you can get custom made wall registers made from wood to fit the opening that is left. I had some made of Douglas fir and mounted in front of the tall baseboard in our 1914 house.
Chamfer the ends of the flooring at a 45 degree angle to keep the opening dimension the same as you lay the floor.

Thanks Pete,
We ended up putting a metal blade in the saw, clamping the registers to a board and cut the 3/4" from the bottoms. They're on and look very nice. I may use alum tape to cover the ends of the floorboards so that they are not always in the direct flow of the heated/cooled air.