Will an 8 inch dual fit? A 7 inch dual on a car that size may be marginal.
Will this be an under floor mount? If so, you will need a 2 psi residual pressure valve for disc brakes when or if you eventually go to them. Buy the disc/disc unit and put two in-line 10 psi residual pressure valves in. Then when you do swap over to discs you can pull out the 10 and replace it with a 2. If it will be a firewall mount, you won't need the 2 psi units for the disc, but you can still use the 10 psi units for the drums. Replace the 10 psi units with unions when you go to discs.