replacing bags with springs

So I have a 3600 # ride that I ve' been running bags for about 16 years. The suspension is a DIY mustang II type with tubular A arms. I'm about to go on a mission to Route 66 and I don't want any problems. I want to swap the bags for some springs but don't know what springs I might need. Any help please?

 

Scale all four corners ,
Then scale with driver & average p***enger.
Then go from there Spring rate.
Most reputable companies have Tech Suport online & how to choose
How about coil over shocks ?