Visit race tech’s site: RT - Digital Product Search and first see if the your stock spring is within range for your weight, at 300lbs, I say you are really pushing it without checking the OEM rate.
The BEST and chepest way to do it is take the shock out, and sent it to a suspension specialist like CMD and they will sort it out depends on the type of riding and the rider weight. If you just want the spring done, it is relatively simple; Most likely you can’t do anything with you OEM shock interms of getting it revalve.
Without doing any revalving, a typical shock service is around $100, add the price of the spring if you need to get one.