First off 66 and 67 frames, no matter which body style they are under, are different. 65 and 66 frames are identical by body style 64 are a stand alone. The only difference with a 64 from 65/66 is the lack of a dimple in the front cross member - the dimple allows the changing of a big block starter without raising the motor. 67s are stand alone because the rear spring upper perches are different. Any of the 4 years will interchange for dimensions. There is just the spring issue with the 67.
El Camino frames like those under convertibles have a boxed center section for extra rigidity and support. The difference is in total length. The extra length is behind the rear axle kick up so it is an easy job to shorten an Elky to fit under a vert.
Convertibles use more mounting points than the other styles but the necessary holes are on all frames.
Yes, 66 beaumont will fit on a 66 Elky frame but the difference is 3.27 inches at the back end
Yes a 66 vert will fit on a 67 elky frame minus the 3.27 inches but you'll have to use the 67 springs. Reason being the upper perches are smaller on the 67. 67 springs are tapered in ( tighter wound ) on both the top and bottom while 64 thru 66 are tappered at the bottom and full diameter at the top.
The 65/66s use an upper spring anti-squeak pad between the rear spring and the spring perch welded to the frame.
