Mick, if I were you I'd use the spring you have and not do anything until you have the full weight of the car on the wheels. The Dana 44s didn't have any different spring than the 36's. It was all based on the suspension option....i.e. FE1, Z51, Z07, etc. What year is your rear suspension? What is the code on the end of the spring?
I honestly don't think the 55 convertible builder is going to help you any more than I can. In fact, he'll probably refer you to me . I looked up my records on that frame and he has an early rear with Dana 36. Here's a pic:
If you have the stock rear spring, there is no reason it won't work. The only one I'd probably replace is an early ('84) Z51 with code "BMH", since it's very stiff at 500 lb/in or 214 lb/in at the wheel. However, the C4 autocross guys love them so they're easy to sell....at least in the US.
Any other spring will work. The softest ones are the latest ones, like 94-96. The most common spring codes are "NYR/RCA/RHZ" and they are 228 lb/in springs, or 147 lb/in at the wheels. The softest is "RR" which is a 149 lb/in spring, or 103 lb/in at the wheel per the info I have on them. The stiffest late spring is "NYU" with a rate of 327 lb/in or 191 lb/in at the wheels.
Again, all of these should give the SAME ride height, but a stiffer or softer ride. So I'm pretty confident you can get the ride height you need once the full weight is on the car, unless something else is wrong.