I chopped mine down, hell even added the passenger side fuel door that was cut out from the longbed donor.
The most important thing to do when cutting the frame is to "z" cut the frame, rather than just chop it in a straight line. Measure twice, cut once and weld it up (i also back plated mine). Main reason for doing this is finding a shortbed truck intact is getting harder these days. Longbeds are all over and not as desirable . Long beds for this reason dont sell for as much. Shorter driveline, modify the parking brake cable, or shorten, tuck the extra tail light wires or splice shorter, and as far as the gas tanks...hmmm I didnt need to do anything with mine. My body lines match, the bed sits even, hell most people would never think it started life as a longbed. Its really not that difficult to think outside the "box". The only way to tell is to crawl up underneath as my welds are even with both gas tanks.
I cut 12" out of the frame, and NO...you dont cut 8 inches off the back. If you do, you will really have a shortbed truck.....