I think you can go up to '89 for a T5 and get a mechanical speedo. Seems like I saw a couple '89's when I was looking. Mine is from an '89 Camaro and it's got mechanical speedo. Inliners has a good page on doing a hybrid.
T5 hybrid To make this swap worth your while, an OD of .78 or better is the way to go. Many of the earlier T5's had .86 which is not much of an overdrive. Many of the later ones had .72, .73. Some Camaro/Firebirds had .63. You can use an rpm calculator such as this
rpm calc. to see the difference between .86, .72 and .63 OD's.
You'll also want to take into account what you're gonna be doing with your truck for determining gear ratios. Such as, will you ever be towing or have a bed fully loaded?
What's the ratio in your rear by the way?
-David
