Measuring the cockpit of my 32 roadster and comparing it to the 31 on 32 rails roadster cockpit, they're very close to the same size.
The 31 is 2" shallower in vertical depth and the windshield is approx 1 1/2" to the rear measured at the top.
The rear cockpit rail to dash front at back of cowl is about the same.

Looking at a couple of ways to go here.

1. Get a set of aftermarket top irons for a 32 and cut the single forward horizontal strap on each side so as to bring the front header back to match the 32 windshield posts (chopped 2") that I'm using on the 31.

2. Copy the 32's top irons making the abovementioned forward horizontal strap the correct length to start with.

Making your own top irons doesn't look to be too tough, specially with a set of irons to copy.

One other way I've thought about doing it would be to get the aftermarket oak bows for a 32 and build a welded top.
If you ran a polished stainless strap down to the tab on the cockpit side rail and let the varnished oak bow show, it would look very much like a regular top bow setup.
Assuming you want to go that way.

In any event - if you build your own top - take note that the standard Ford tops overhang the cockpit side rails to an extent.
Makes it easy when you wash the car and also keeps rain out as long as it's not blowing sideways.

A lot of the home-made top frames I see (and many of them are very well done) the top is narrower on the sides than the cockpit side rails & door and rain goes straight in.

As a small fwiw, the 32 windshield posts are easy to adapt to the 30-31 A stanchions and making your own 32 style windshield frame isn't too difficult either.
The combination of 32 windshield posts, 32 style windshield frame (rectangular instead of round tubing like the A's) and a 32 top make for a very nice looking car imho.