if I put in a 1/2 ton rear and tranny would I get up to the speeds I want? would be great to keep the 3/4 ton capacity
You need to figure out what gear ratio will work best for you. Google "rpm calculator" and play around with some variables.
You have a lot of options for a six lug rearend because of your truck being a flatbed, as width is not as much of a concern with no fenders.
I would say your 3/4 ton capacity is already diminished due to your 6 lug front axle.
