Any drawback to keeping the drum spindles on my 66 Valiant?

Clifcuda65’s pictures tell a good story. Whatever brake setup you go with, mount the calipers on the rear so you have clearance for a front sway bar. The only drawback that I’ve experienced with the 73-76 LBP discs on an early a-body is the 5/8” increase in track width on each side. Dr. Diff has a really good deal on complete 73-76 disk conversions for about $450. Another option is Scarebird that uses your existing spindles while mixing readily available components from newer cars.