Nice thing about a 6.5, decent fuel mileage. Cheap to work on. Problems are known, most are very easily fixed. Power, yeah a bit on the under stated side compared to todays motors. STILL, a 200/400 motor is nothing to sneeze at, and they will overall pull upwards of 8-10K without issues at reasonably decent speeds etc. Keeping them cool is the main issue.
I went from the pre97 cooling mods, to the post 97 with dual t-stats, high gpm pump that is about double the water flow, and a 7 instead of a 5 blade fan with a different fan clutch, made a lot of difference for me, IE I could use the AC vs not and still have lower engine temps.
Marty