Sorry guys but he asked about a specific aluminum radiator...it will not fit! I know, I own one. The original radiator is NOT a 26" like the b-bodies. As you say the original, also in mine, is actually a 25 5/8" wide and a little shorter. You can get an aftermarket crossflow radiator that is 26" wide to fit as you say. The original "26 inch" is a 2 core and won't cool my 400hp 360 here in the desert heat.
in my 68 fastback , could go a hair wider w/ some work----------