do i need a new driveshaft??

Im putting a 350 into my 87 s-10 extnded cab 2wd and im puting a th350 behind it. am i going to have to get a custom driveshaft or is there one from another car that will fit, ive never done this before so if anyone has donethe samething im trying to do any help or pointers would be great, thanks

Yes, you will need a new driveshaft, custom cut to your measurements.

I used a one-piece driveshaft from a Chevy 4X4.

Make sure you use a one piece shaft and not a two piece (like the one you have now in the extended cab) one less joint to break in the future. Mount your motor and trans then measure from the center of the U-joint in the trans yoke to the center of the U-joint in the rear yoke (but check with your machine shop for the measurements they need). Get a transmission yoke from a TH350 trans and take the rear yoke from your existing driveshaft.
Good luck !!!!

Toenail, I forgot to tell you, my mechanic friend recomended that if you want to beef up your rear diff look into getting a Auburn locker. Apparently they make some sort of drop in unit for the S-10.
He also sayes that your diff should be fine unless you run very high horsepower. He ran a 500+ hp motor for years on one with slicks on without a problem