looks like a fun project. i'm sure you will get something together that will work.
i'm just not sure calling it a "grader" is the right term for this type of implement. the key to a road grader's ability to smooth a road is it's long, rigid, wheelbase in the direction of the road. it has a fixed blade with the ability to apply a significant amount of downforce (tons) and the ability to be angled precisely. the geometry of the blade combined with the long wheelbase allows a skilled operator to recontour a road base with essentially no effect from the defects in the road originally present (pot holes, wash boarding, washouts, incorrect banking, etc.)
i believe what you are building and what many (landpride, dr, etc.) sell as a road grader is really a fancy back blade. it will not have either of the key benefits of a road grader (long wheel base nor high adjustability with lots of downforce).
the wheel base will be the same as the tractor's without the blade on. the tractor will still ride into every imperfection in the road. the best you can do to minimize this is make the 3pt hitch "floating" as you plan to do.
if you allow the 3pt to float, you will not have any adjustability there. you will have to build several types of adjustability into the implement itself (blade angle left to right, front to back, yaw and pitch, etc.) this will be an engineering challenge but also the key to success. adding gauge wheels on long mounting brackets will allow you to add adjustability as well as lengthen the effective wheelbase of the implement. essentially, you would be building a tow behind road grader like they used to use in the 20's and 30s.
you will not be able to apply downforce. referencing your question, here, weight is your friend although it's a compromise since the heavier the implement, the harder it is to hook up and the less dirt you can move before loosing traction of the drive wheels.
not trying to be a wet blanket. i love a good project as much as the next guy. but, my parents have also lived on a 1/2 mile gravel road for almost 30 years. i've seen most of it and tried most of it. nothing beats the design of a real road grader for taking care of things in short order. everything else is a compromise.
i run a six foot rear blade with extra weight on a 2 ton tractor with adjustable tilt. by angling the blade all the way and tilting it, i am able to crown and get out most wash boards when the ground is moist. i just started digging out pot holes with a 50hp gear box tiller. that is the best combo i've been able to come up with short of buying (or building) a road grader.
sorry for the long post and good luck on your project. it will be interesting to see how it all turns out!
amp
