My experience is that 2200~2300 rpm is the perfect cruising rpm for the 235 in an AD truck. On flat road without wind, you can drop that to 2100 or maybe a bit less. With a 3.55 and 27 inch tires and driving 50~52 mph, that is exactly where you are...2200 rpm. And that's why your engine is saying "I'm good right here." With this in mind, let's work backwards from there and see what you have. That .68 fifth gear will be challenging...no questions about that, but with the 3.73 rear and 26 inch tires, you'll be spinning a little over 2100 rpm at 65. That is probably workable, but you need to understand in wind and hills you'll be downshifting to 4th. And...another positive...your starting line ratio if you have the 3.27 first gear (which was common in transmissions with the .68 fifth gear) will be 12. Which for a 1/2 ton pickup is perfect. Good luck.
