They should be. The tank on my '93 looks exactly like the ones on my '83 and '87....35 gal...I always thought that was large for a gasser, 'til I pulled the 318 and slid in a 440...Here's my "walk through", with a lot of ancillary worthless stuff that isn't relevant.
http://www.cumminsforum.com/forum/8...ossibly-dumbest-question-ever-asked-here.html
The Ramcharger tank fits in where the spare tire goes, I don't know if saddle tanks off of gas pick-ups would be bolt-ins.