I'm running a 2.95 1st gear T5 "Z" hybrid behind my built 355 cid small block with a 3.73 R&P and a 28" tall rear tire in my streetrod and my 5th gear is a .63. Using your numbers and assuming your 2.95 1st gear T5 is also a .63 you would be turning 1692 rpms at 70 mph in 5th gear.
Based on my experience I'm afraid that your 5th gear would be of little use. (once again, assuming that your 5th gear ratio is .63) The 31" tire diameter and the 3.54 would have you coming into the most efficient section of your power curve at speeds well above the legal speed limit and would have you lugging down your engine and requiring downshifts at or below 60 mph.
My combo (.63,28"3.73,2850 lb) lets me drop down to 50 mph without a downshift but a 4.10-4.11 would let my engine operate deeper into the power curve more efficiently. With the heavier 4X4 a 4.56 which would only give you 2180 at 70 mph....very shallow in the power/torque curve for the small block.
If you plug in your numbers using the CSG calculator mentioned by "Grandpastruck" and you confirm the OD ratio of your T5 you should be able to come up with some numbers that would put you in the power curve of the SBC. Being that both your front and rear R&P ratios in your differentials are the same, it would appear to me that the least offensive way to get your engine spinning efficiently is to get your transmission final drive ratio numerically higher or to drive the truck as a 4 speed with limited use on the highway in OD.
Once again.....this info is based on the 2.95 T5 with a .63 OD. If by chance you have a different OD ratio you can plug in the numbers and make your own call. I hope this helps.
Dave
