Good morning and welcome to Stovebolt!
Our resident gurus will be along soon with some ideas for you. In the meantime, though ...
Right off the bat, let me deliver the cold, hard reality slap here ... You can do some gearing changes, transmission swaps etc, etc.... but in my opinion, the 216 just doesn't have enough power/torque/whatever to do freeway speeds. If you want to comfortably achieve/maintain freeway speeds, start thinking about an engine swap.
There will be folks who will tell you about add-on overdrives, t-5 trannies, etc, etc, etc all day long. But at the end of that day, you will still have an engine insufficient for the task. The venerable 216 can huff and puff but it simply can not blow that house down. Show me someone who sez it can and I'll show you someone who is constantly shifting and messing with an overdrive.
Been there, done that (even started this web site ...) tried to live the 216 dream .... twice! Which is why my truck has a 261 in it now.
Just sayin'
John
