I have dynamic 5000 stall mechnaical diode 8” converter in my car. Drivability is better than I ever expected. When I sit in traffic and remove my foot from brake pedal, the car slowly creeps forward. It’s manual shift car and it actually behaves better with this converter than my previous combo...
I know a guy running 9s in Dart with 8 3/4" foot brake car but its pushing it.
I have the same dillema as you and am leaning towards building Dana 60...
You can put billet caps on 489 but there will be some cost of machining involved.
In my case it will be with 1.6 ratio rockers meaning .682 lift before lash, but my heads flow nice at .700 so I wated to take advantage of that without some crazy cam profile. My car gets a lot of street time and I have 3500 stall converter.