I've adjusted about a dozen SMB penthouse tops (all econolines) and there have been two types of helper springs.....they are the same except for length.
Several of the EB body tops I have adjusted have 1 ft long springs; my RB had 2 ft long springs, otherwise everything is the same.
The EB's with the shorter (1 foot long) springs worked far better than my 2 ft long springs, likely because the spring rate is double.
helper springs in EB body that work great:
.250" wire dia
11.75" Free length
rate 52 lbs/in
23 active coils
My springs (RB body):
.250 wire diameter
24" free length
rate 20 lbs/in
47 active coils
I have replaced my helper springs with a 3 stage gas spring arrangement.....it works better than the helper springs but I have to replace leaky gas springs every so often.
This thread should be of interest....starting here: