I don't think that comparing a 216 motor to a last generation 235 is a fair comparison. Its a bit of apples vs. oranges. As to how hard you can run an engine has many variables that relate to each engine. If you look at the driving conditions when the 216 motors were sold and then look at the driving conditions of the early 60's you will find 2 different situations. Engine condition also plays a big part of how hard it can be run and how it will perform. Your '48 3800 should be able to run 40-50mph with a 216 that is in good tuned condition. A later 235 may run some faster. You should also consider the brakes and steering before you get out on the road and wind your truck up to cruising speed because sooner or later you need to stop and you also have to keep it between the lines on the road.
