If you already have 3.55 gears................................
THEN LEAVE IT ALONE!
Add gas and drive.
You can certainly change out to an open rear and then have a new driveshaft made-hopefully it balances up on the first shot. And then you need to change the transmission, or at least the rear of it. So exactly how much money did you want to spend to go backwards? Have you actually driven your truck yet? What is the issue? You should be able to do 70mph with that gearing pretty easily. I know a few guys with 3.55's who have run them up over 90mph with your exact setup. The 216 may not like super high revs so 70mph should be plenty. Give it a drive and let us know how it did.
