From front side look at the bottom two center retaining nuts, if a socket cannot access them to remove, only a wrench, then it is a 9� See photo below.
I assume that I can just swap the coil spring and retain my existing shock assembly if need be?
Yes, you should be able to retain your existing shocks, you'll just need to adjust the coil portion of the coilover. This is exactly where I'm at. To accurately determine the correct coil rating, the rear of the car should be weighed. The weight of the rear of most muscle cars is known by the aftermarket coilover manufacturers, but my '66 is heavily modified and the weight is an unknown. Your rat rod is going to be similar. In my car, there's no spare tire, but the battery is back there. The leafs are gone with the coilover conversion, but that's unsprung weight and technically shouldn't matter. My back seat is removed and replaced with a FG package tray, but I have a one-piece frame using 2x3" boxed steel channel. I have no idea how much the rear of the car weighs, but I'm guessing "lighter". If I buy a set of 250# springs and they're too soft... I'll get into the "keep buying differently rated springs until I find the right one" game.
You should be able to call the manufacturer of your coilovers and inquire about numerically lower rated springs. The factors to consider are: compressed shock length, fully extended shock length and then weight of the rear of the vehicle. This is probably something you're going to have to do after the bed, rear bumper and other rear weight pieces are installed.
or if you need custom springs made, there is a place just up the road from our shop in St.Marys!
This may actually be your best bet. From where I'm sitting it's difficult to determine the exact weight rated coil spring without buying a darn assortment of them... kinda like buying differently rated leaf springs to get that "perfect" ride height. With anything other than a somewhat original (weighted) car, it's a crapshoot. If you could drop the
car rat rod off at a shop and say "make me a set of coil springs that give me an X ride height and a Y firmness"... then the oness is on them to figure out the correct weight, something that they're likely more apt to do anyway.
