Ford quit using a straight axle on vans in 69, Chevy and Dodge in 70. None had disc brakes from the factory. Ford did use disc brakes on the twin I beam sometime in the mid 70's, Chevy and Dodge went to a IFS with discs about the same time. There might have been some 3/4 tons in the late 70's with discs on a straight axle, but I couldn't find any evidence of them. My 87 E350 based motorhome has discs, but it's a twin I beam. I think all Chevy and Dodges from 70 on had the IFS reguardless of the weight rating.
So, if you want a straight axle and discs, you will probably need a conversion kit from Speedway or somebody like that.....
As to a rear with discs, lots of late model Crown Vics with a 8.8 have discs. Losts easier to find than a 70's model rear with discs, and you can adapt the discs from them to Ford 8" and 9 " pretty easily.