Sounds like your interested in putting miles on your truck and I would highly recommend the S10 .72 OD mechanical speedo T5. As Woody pointed out, retaining the SM420 is going to give you a bulletproof box that would be great for local driving. The T5 would expand the versatility of your truck. However, there's more to it than just bolting on the transmission.
If you haven't already upgraded your brake system you'll need to give that SERIOUS consideration, as well as making sure that your front suspension/tires are ready to roll along at 70mph. The 3.07 diff you're currently running would definitely be too tall for the .72 T5 so you'd have to consider the .86 T5 found behind the 4 cylinder S10 engines. Also a good transmission, but it has a 4.03 1st gear, which has you short-shifting almost immediately.
I'm currently running a .72 Mechanical speedo T5 with a 3.42 diff and my cruise is 2000 rpms at 70 mph. Great for longer trips but puts me on the lower end of the torque curve when putzing along at 50-60 mph. A 3.73 would be better for maximizing engine efficiency/mileage. My engine will accelerate from 45 mph in 5th gear with traffic, without a downshift and will easily cruise at 75-80 with rpms to spare.
The modifications to the T5 are modest but you'd be wise to consider the total cost/work involved in upgrading your truck for today's open road speeds. I documented my upgrades in my webshots albums and this might give you some perspective of the process and the end results.
Dave
