I never found a frame from a later vehicle that had the same wheelbase as a 65 El Camino. If you don't mind cutting and rewelding, I would use an s-10 pickup frame that has either the long bed or extended cab. I believe the wb is a 118 inches. I believe the El Camino is 115 inches. So you need to lose three inches somewhere in between. The great benefit would the amount aftermarket parts for suspension and brake upgrades.
I was planning on the same thing, but did not pursue it futher due to house project and limited space and funds. Good luck.
I just bought a 65 Chevelle El-Camino frame from Texas. The frame ran me $100 plus shipping (about $200). The frame is spotless. I would begin my search in the southern states (Texas, Arizona, New Mexico) and stick with an original frame.
Hope this helps..
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