A cable driven speedo is the best to have. Maybe you can get lucky and your original cable will be long enough to reach the spot on the T-5 transmission. Most likely, though, you will have to get a ratio adapter.
I used the 85-86 S-10 T-5. The ratios are work well in my truck with a 3.54:1 rear axle. I have the .72 overdrive gear. I don't think the transmission holes are going to match your bellhousing. I believe you will need an adapter. One source is Steve at Buffalo Enterprises. I would recommend a 1/2" thick adapter. It should work well and when measuring the various distances for the input shaft length and throw out bearing retainer length, the 1/2" length is easy to work with. Yes, you will have to drill out the transmission mounting holes to 1/2". The clutch and pressure plate you use will depend on the diameter of the clutch your flywheel will accept. The RC 4190 is the under 9" clutch, the RC 4201 is the 9 7/8" clutch and the 4212 is the 11" clutch. While I'm not that familiar with the other size's numbers, they fall in that same part number range. With the 85-86 S-10 T-5 transmission I'm talking about, you will have to have a 14 spline clutch and that's the way you'll have to approach what kind of clutch you will need - by spline and size. With that information at hand, your provider will give you the proper pressure plate.
It is a great thing to do for your truck. You'll like what you end up with.
Good luck!
