Hey Dan,
In regards to the speed increase, the best bet would be to find an overdrive to mount behind the new engine. If you are still serching for an engine look for one with an overdrvie in place, then all you would need to do maount the shifters for it. Updating to a later model 261 or GMC 270 would be a good bet, especialy if you plan to drive it at higher speed, the oiling and bearing setup in the 228 is nowhere near as strong as a late model. Lastly the gearing in the Dana 70 might be higher than the original axle, this might give you a slight boost in crusing speed as well. Talk to the guys at the shop that is doing the work, and they might be able to give you an idea on drivetrain upgrades that might help as well, especialy if you are not set in stone what you want to do yet.
-Jason
