Is there a such thing as an aftermarket speedo that is adjustable to any ratio? Should I convert to electric? What are my options that don't involve dissembling the tranny?

The search button is a great thing, there's been some great write ups on how to do this. Its basically just pulling the tail shaft housing off of the tranny and swapping the speedo gears out. I believe there is a write up that tells you what gears to buy to correct the speedo on here.

Yes, you can get satnav-based speedometers. AutoMeter makes some.
If you have a pre-'89 with a cable speedometer, then transmissioncenter.net can sell you a little inline corrector gearbox for $62.

do some math to find your rpm at 60 (4th gear in a T5, 3rd gear in a 70r4).. divide that by 1000 to get your final drive ratio. now you just need to find a combination of drive and driven gears that comes close to that ratio- sometimes you can hit it right on, sometimes they are just a little off..it's not hard, but you do need to do some really simple math... there is probably a set of factory gears for a T5, since they put 4.10 gears in some 4 cylinder/5 speed S series trucks...

google is your friend.