You've probably already done this but you will want to measure the depth - width - length you have under your truck for the tank and compare that with the S10 tanks overall dimensions.
These links have dimensions of S-10 Gas Tanks:
http://raybuck.com/i-66692-1985-95-s...ank-20gal.html
http://www.justgastanks.com/store/in...Path=67_84_294
http://monsterautoparts.com/CHEVROLE..._gas_tanks.htm
Looks like they are pretty long....
Crossmember interferences will likely be an issue...
If the tank is too deep it might look nasty hanging below the frame rail..
Good luck and let us know how it goes!