Yeah, I knew the 80's pickup side-saddle (outside frame) tanks wouldn't work, but the newer Tahoe and a few others mount a single long tank inside of the frame, to the left of the driveshaft with cat/exhaust all down the right side. The Tahoe also happens to use the same "cable winch" spare design the Toyotas do. Slim chance the Tahoe 26 gallon plastic tank would fit in that same space on the early blazer, since the late model plastic tanks are molded to conform to specific floorpans/crossmembers, but a custom aluminum/steel fuel tank might be a better option anyway, since it could be tailored to the 69-72 floorpan, maximizing capacity. Of course this all assumes there's enough room to hoist a 32-35" tire up between the rear frame rails b/n the axle and crossmember. Thought I'd throw it out there to see if anyone ever looked into it.
Here's a closer look at the Tahoe setup;