70 C30 dually combined towing capacity GCWR?

[jarrodblake]

I am trying to calculate the combined towing capacity for my 70 C30. The truck has a maximum gvw rating of 14,000lbs. The truck is a dovetail flatbed truck. the bed is 13 feet with a 4 foot dove tail. My plan is to make the dove tail removible and add a hidden GooseNeck hitch. I would like to be able to pull a GooseNeck camper. Or even a flatbed GooseNeck trailer with a small camper and a vehicle on it. However I do not know what size trailer I can pull or how to calculate it.

I know modification do not increase factory towing capacity ratings. But I am in the middle of converting it to disk brakes all around. hydro boost and air suspension. The truck already has a trailer brake controller but I plan on upgrading it. I also have a nv4500 for it.

I have read a few threads but they are all dead ends. I can not find anything on google it looks like all chevys until like 89 did not have a GCWR.