I wire wheeled the under side of my 71, just like you, then applied POR15 from the torque boxes back and that was it. I did apply some spray on undercoating in the rear wheel wells. The front clip was painted with primer and then catalyzed satin black enamel. This was in 2003-ish.
FWIW, the POR15 paint that everyone uses is not a rust converter, it's a moisture cured urethane paint. They recommend their MetalReady beforehand, which is a phosphoric acid based rust remover / converter. I've found the MR to be very handy to keep around for lots of general rust removal and prep tasks.