The runners on '79 and '86 beds I have both measured 1.5", while the '89-95's are 1.75 wide at the base. An 1/8" difference per side isn't that much, however, the pre-'89 are slopes are a little steeper than the '89 and up. You could but weld the pre'89 section in and it wouldn't look to bad, most probably wouldn't notice the difference, but if you want them to overlap, it wont work to well.
The 1st gen Tacoma might be the same, as well as the T100 and 1st gen Tundra, so you may not be limited to just the '89-95 3rd gen models.
