I don't know why you would want to do it to begin with. The heat strips in most AC units over the years have been known to be marginal in heat output and require the noisy AC fan to push the heat out. And of course it is at the ceiling, while the cold air accumulates at the floor.
Armstrong AC units have not been made in many years. I would guess that if you want to go with this project, it would be a custom home made job.
Why not just get a nice portable heater of the 1500
watt capacity, which is all the heat strips do to begin with?