I am not sure the opening of the core support will allow full face cooling for a 26" radiator. I just looked at mine and a 22 core fits the opening almost perfectly with maybe a 1/2 of frame on either side.
Actually that depends on what style radiator you use. If you stay with a stock type that has top and bottom tanks your correct, the opening isn't big enough. If you go with an aftermarket type with side tanks they measure their width counting the tanks so the actual core size ends up only being about 23-23.75". I've been researching putting a bigger radiator in my 68 Cuda and it looks like the 26" wide crossflow will work fine because my core opening is approx. 23.75".
