Found a possible donor car. Just want to know will the dash from a 73 4door dart fit a 74 2 door Scamp? And how about the fuel tank and sending unit.

Everything will interchange except the doors and glass... (so: dash, wiring harness, suspension, drivetrain, fenders, hood, exhaust, fuel tank, sending unit, bumpers, etc...)

Be aware though that the later dashboards like this one will have the VIN rosette riveted to the dash and there isn't a legal way to transfer that between the two (if just worried about the dash pad, then no issue here...).
If you transplant the dash (remember that that the windsheild needs to come out...), you may need to talk to your local about the VIN issue. Here in Oregon if you have the title for two cars (i.e. I transplant the dash and driver's door from a valiant to a dart that have different VIN tagging from the basis car), then I can have them combine the VIN's to one title and registration so the state police don't collar me for 'chopshoppping' a vehicle.

-D.Idiot