The 2-door and 4-door frames are not the same. Every dimension is identical except the distance between mounts #2 & #4: The 4-door frames are about 4 inches longer between these mounts. All other dimensions, including the distance from mount #4 to the tail of the frame (mount 7) is identical between the 2-door and 4-door.
If you have a really screwed up 2-door frame, and have found a perfect 4-door frame, there is nothing wrong with doing a little alteration: You can use a plasma arc or a swazall to simply cut a section out of the frame and shorten in to the 2-door spec. Weld it back together with a well-prepped CJP weld and you'll have a frame just as stong as an original 2-door frame. I've even used the long El Camino frame to create a "correct" 2-door A-Body frame that fit perfectly (the station wagon and El Camino frames are longer both between mounts 2 & 4 and from 4 to 7, so you have to make 2 cuts). No problem! Make accurate measurmenets and cuts, do a good double-bevel weld from both sides, grind flush, and you'll have a perfect "original" 2-door frame.
Here's some creative cutting, fabrication, and welding of a Corvette Frame in my workshop. Doing a frame repair is no issue: Take your time and just dive into it: Anything can be fixed!
Chopping a 2-foot section out of the frame. Stiffening bars have been welded into place to hold the frame in correct alignment while the cutting and welding is being done:
New metal and fabricated pieces welded into position:
Finished weld section looks better than factory: