You dont really need to throw a pcs in if doing the conversion right. The sct with proper tuning will control the trans just fine. It sounds like he needs better trans tuning on the sct. We use Diesel Conversion Specialists and their tuning is spot on and they email you updates if its not right...
Save up for a Hydra Chip from PHP. Having the ability to load tunes and not have to send it back and forth for updates and being stuck with one tuner like other internet update able chips on the market is an awesome feature especially with doing custom tuning.
Personally I love everything about PHP tuning, I have a 7.3 with a man pedal though. I have heard great things about Gearheads tuning as well as PHPs 6.0L Tuning. Id give both of those a shot.
Im running an s366 with 250/100s for an option for you, havent towed with it heavy but light trailers and loads it moves just fine. Be interested to see how it tows heavy in the spring when I hook up to the goose neck.