sfbd exhaust...

i have stock floored a few 99-06 gm trucks to the rockers but i am doing one that is bd 5 inches and now im having trouble running my cats ... do the have to be mounted the same distance as they were stock or could i move them back and add about 12inches of exhaust?

anyone ever ran into this?

Totally diff truck but on a 94 yota I put the cat just in front of the rearend and it passed smog no problem. I was told wrapping the exhaust in thermal wrap would keep heat in, getting the cat hotter and keeping the floor from getting too hot, I guess it worked. Stock the cat was under the driver. Good luck.