Keith,
What I'm saying is that carry on makes a 5x8 no floor as a standard product - TSC probably doesn't stock it and it may be special order from TSC. Others have purchased it. I beleive the extra dollars for the 5x8 would surely outweigh modifying a 4x8 as I think it is only about $100 or so more. Call your TSC store, tell them you want to order a 5x8 just like the 4x8.
Red trailers has a 5x8 too
http://www.redtrailers.com/ShowTrailer.asp?id=8
Others have used the HF 5x8 assembled without side rails
http://ww2.harborfreight.com/cpi/ctaf/D ... mber=93747
All of these choices may make more economic sense than modifying a 4x8 and be less expensive than custom made.
Personally, if I was to get a welder and modify a ready made trailer, I would rather start from scratch and build what I really wanted. I think you will be much better off that way. There is a nice tutorial in the chasis section.
Mark