71 LWB towing options

[Shon]

I have a 71 LWB with Coil springs. I am going to be swapping to a 6.0/4L80 to tow my trailer/car; figuring 4700-5000#s loaded up.
I have air shocks on there and loaded it up this weekend and the trailer was level and the truck was squating some, if I put a level on the bed it needed like an inch to be level; I did move the car back some on the trailer but it did not make much difference. I have a pic of it I can post later if that will help but I am not sure the air shocks are going to be enough once we start moving with the dips in the road or any additional stuff in the bed. shocks aired up to 120PSI, the max my compressor will put out.

I am looking at options to add to the air shocks, or if need be I can remove.
The ideas i have come up with are:
1) Helper spring added on
2) Air bags in both springs
3) HD springs

What do you think would be best?