Cold Case 26in Rad fitment

I can't comment on the 26" fitment but its not overly uncommon to have to fit most brands of radiator. The 70-up 22" Cold Case I installed in my 69 fell right in, the Champion I had before that did not.

For the stock shroud I rived on some thick, black, rubber garage door seal that had a lip I biased against the radiator tanks at the top and bottom to seal the gaps. Seems to work well.

The thicker core radiators stick out far more that stock. So you will need to fit your fan accordingly. I think I have the same 2-3/4" space you had and ended up running a 18" flex fan. But I have power steering so there is lees room for me to fit a fan between the core and the PS pulley.