Fitting a 26 inch radiator?

That’s not true.

The 26” radiator will fit any 67-76 A body radiator support, but it’s not just a drop in. The opening in the radiator support is not as wide for the smaller radiators as it is for the 26”, so if you install a 26” radiator some of the radiator will be behind the support. It’s not very much, so when I did that swap I didn’t open the radiator support any. But the 26” radiator support opening is wider by about 2" per side.

The 26” radiators also have wider brackets, so the radiator will not line up with the mounting holes in the radiator support if your car had a smaller radiator from the factory. The mounting holes for the 26” radiator are a few inches wider. I welded new mounts onto my radiator support when I swapped my /6 Duster over to a 26” radiator.

So a 26” radiator will fit, you don’t need to open up the radiator shroud but the factory 26” radiator supports did have a wider opening, and you will need to drill new holes to mount the 26” radiator to the support if the car was originally equipped with a smaller radiator.

***edited for clarity on the dimensions***

My mistake, limited personal experience strikes again lol. I haven't messed with any slant-6 A-bodies myself, my Duster was originally a 318 car with A/C and came with the 22" 3-row radiator. I got an aluminum 26" radiator from ECP and it was a pretty easy swap, just had to massage the lower support a bit with a big hammer and 2x4 chunk of wood and drill 2 holes in the flanges on the new rad to match up with the factory bolt holes.

Engineered Cooling Products is great, their radiators have 2 rows of 1" wide cores (more surface area than a 3-row) and very competitive pricing. They also have highly detailed drawings on their site with all of the dimensions, bolt hole locations etc clearly shown.