When I first put my 454 in my suburban it would detonate when towing, I retarded the timing a little bit and it got worse, so I advanced it and so far its seems to like 8 deg adv
Not sure if there still in business but I had bobs transmissions build 2 for me when I lived down there. Last one built in 2003 and it just gave up last month.
Sounds like you need to find a better trans shop. I have a 700r behind a 454 and tow 5k lbs on a regular basis with no issues. They can be great transmissions if built right.