'76 400 in a '73 duster
I have a '78-400 in a 71 Duster. Added on top is a small TQ, Holley Street Dominator, B body headmen headers into 2-1/2 exhaust/Hooker Aero Chambers. A MSD fires off a MP distributor. Aluminum rad and electric fans.
So I know a little of the road you travel.
Your problem is the ultra low compression of the engine. Adding a cam will further hurt this issue adding up to less torque.
You need to raise the compression and use a better cam. Add a TF1 shift kit and a higher stall converter.