If you want all of the door, hood, and tailgate gaps perfect (3/16") then not only should you build it on the new frame, you should completely assemble it before paint. That includes the windshield frame, grill, and hardtop if your using one. The hood gap is really a work of art to get right. Has to be perfect along the sides and match side to side, perfect at "T" where it intersects grill, perfect along back side where the hinges are (can use shims a bit), and perhaps most difficult is the height of the hood bow down each side of the fender is often not the same as the fender bow. Fixing that can be difficult. So yes, new frame, make sure you completly understand how the frame to body shims work for gap adjustments, and plan on spending two or three times longer getting gaps perfect than you think.
