The 3L UIM will not bolt up to the 2.5 LIM without modification (if possible at all), I do know that someone has put 2.5 heads on a 3L and used the 3L upper and lower IM's
Have you tried the SVT upper on your cougar?
I beleive Warmonger and Terry have posted info on port sizes in the UIM (3L IIRC is bigger), try a search in 3L section and you should come a cross a few posts with that specific info.
