Re: my 74 B
The problem isn't the brightness of the bulbs, it's the electrical circuit. You need to check two things before touching anything else.
First, disconnect the wires from the rheostat (dimmer knob) and connect them together. If the lights get noticeably brighter then the rheostat is bad and must either be rebuilt or replaced.
Next, remove and clean all grounds. This includes removing each light bulb and socket and cleaning the socket with emory cloth / sand paper. Also clean the surface which the socket connects to.
Consider putting a bit of dielectric grease over the ground connections once they are cleaned and reattached. This will help prevent corrosion from setting in again.
Once you have done this, the lights should be much brighter as they were designed to be. there is no other way around the problem other than installing brighter bulbs which puts more of a strain of the system, and eventually they too will dim as the system gets older and more corroded.