Percentage GC is just a guide. The important aspects of pcr primers are similar annealing temperatures and no homology at the 3' ends to avoid primer dimer. For long amplimers keep the annealing temperature high to avoid high GC regions forming secondary structures. I would make primers both annealing at about 65c which do not form primer dimers and sort out any amplification problems later

I agree with Paul Rutland

I would not take such designing rules as too serious and would just try to design Primers with Primer3 with the normal parameters, if this is not possible, go down with the stringency of the parameters.

I would design 2 or 3 FWD and REV Primers, combine each of them with each other. By this you have the chance to get a good PCR product in a quick and cheap way.