That depends on what you mean by "longbed."
The 3600 trucks had a 7ft long bed, wheelbase was 125.5.
The 3800 trucks had a 9ft long bed, wheelbase was 137.
I believe there were some early 3100 trucks with the 7 ft bed like the 3600's
So, if you go with S10, you'll need at least an extended cab, short bed model. Even then you'll have to stretch it. They were only 122 wheel base.
Another option is the Chevy Colorado, the extended cab is 126. Which is perfect for the trucks with the 7 ft bed. Most folks move the rear axle back slightly anyway, to center the axle in the fenderwell better.
Something else to consider, which is growing in popularity is a Trailblazer chassis swap. The regular TB is 113, and the EXT is 129.
Here's a good link
http://www.vortec4200.com/forum1/vie...php?f=10&t=939