Rear Spring Comparison

[Spike 72]

I have a 72 GMC shortbed. I installed new 5" coil drop springs from LMC. They ride pretty well but the rear end is very spongy & I cannot put much weight in the back. Has anyone else had this problem? If so, what did you do to fix it? POL springs (Performance Online), Belltech, Hotchkis, these are all double the price. Is it because they are progressive springs & the LMC one are not progressive? or has anyone installed the airbag helper springs? If so, how do you like them & how did it change the ride? thank you for the help.