I took my radiator to a local radiator shop (two of them before I found one who would do what I wanted).. and described what I needed them to do; which was 1) to dis-assemble my radiator for me including removing the top/bottom rails from the core, 2) I picked up the parts, and cleaned all the parts I was going to reuse (side tanks, tag, brackets, and top and bottom rails); the core company requires that the rails be 'clean metal, no paint or rust., and 3) I shipped off the rails to the core company after first *ordering a '69 repro radiator core, with qualified notes to use the rails I would send them, 4) the 'custom core' was sent to the radiator shop for reinstalling with my tanks/etc. I held onto all my critical parts until the core was returned and the shop called me (for fear damage might occur if they laid around the shop for a couple of weeks). Three days later I picked up my redone radiator, minus paint.. because I wanted to paint it myself.
The radiator shop told me that they only had to heat up the rails all the way across to remove from the core... (each little fin is soldered to the rails)..
they said 'no problem' for them, but I have no idea how I would have done it..