TheShoer - I think
Eaton Detroit Spring is probably the most widely known source for new replacement springs for our old trucks. There are other suppliers, but not sure how much luck you will have with something not original specification for your truck. I believe they will make springs to order based on your needs, but you will need to contact them and provide details on your project plans and dimensions. Aside for the length and width dimensions (also load range expected), you will need to know or figure out the arch height of your springs. You obviously are going to need extra clearance for a front wheel drive axle in place if the original steering axle and perhaps extra load capacity depending on your powerplant/transmission choice.
Others will chime in soon, but I think this may give you a starting point.
