I would reconsider using a different gear ratio. With a 3:08 you will have a hard time maintaining your speed on a flat road with either OD ratio T-5. A 3:42 would be a minimum that I would use and a 3:73 would be the perfect choice in my opinion.
I think 0.86 OD would be the only option with a 3.08 rear. Depending on driving conditions and engine performance, you are correct, it might bog the engine.
Calculations show 2066 RPMs at 65MPH with a 28" tire and 3.08 rear. That should work, theoretically. A 29" tire would give 1995 RPMs.
I'd recommend swapping the tranny first. Drive and see how well the combo works. If 5th gear bogs, then just drive it like a 4 speed and get a 3.42 rear later.
