From my 2000 service manual I find that the overall length of the 134" and 138" wheelbase frames is the same. These wheelbases correspond to the regular cab, long bed and the quad cab, short bed, respectively.
The main difference is in the mounting points for the cab, which is what you expected. The distance from the front reference point to the rear frame crossmember is also different between the two cab styles.
My guess is that if you are handy and don't mind welding or riveting to your frame, you can make either of these frames work, but if you get the regular cab, long bed frame, you'll have to move some items that are in the wrong places. If it were me, I wouldn't want to weld on my frame, and I don't have the facilities to do a proper rivet job, so I'd find a frame that matched.
Aside: You don't say where you are located. If you put your city and state into your profile, other members may volunteer to help out. If you are near the Cincinnati area, you can come here (in Indiana) and check my manual.
Another resource to check for local help is the TDR Companion. It isn't intended for this sort of thing, but I'm sure that most of the listed members will be happy to lend a hand.
Good luck,
Loren