[JM 35 Sedan]

I thought I would add my 2 pennies on this.
First, if the '36 steering box is built, adjusted and lubed correctly, it is a good box that is not all that difficult to steer with. JMHO
As much as I respect Vern Tardel, I agree with Tom on drilling an extra hole in the top lug of the '37 to '39 steering box. We recently tried doing this on a stock '36 Phaeton and the later box interfered with the left side exhaust manifold. There was no other position on that '36 frame bracket, that we could find, to mount that later box that would give a three bolt mounting with a correct steering column mounting at dash. So at that point we began removal of the '36 mounting bracket to bolt in a '37-'40 bracket. We used a Bob Drake repro bracket that is an identical copy of the original. Removing the '36 bracket on a completely assembled car was a real challenge. The rivets were a SOB to remove and that one weld was a bear to break loose. Once that nasty job was completed the new bracket and complete '37 steering column bolted right in. This car does steer really well now.
I would strongly suggest that if your car is apart and you plan to do this conversion, remove that old bracket before you start putting the car back together.
One thing to consider though, a knowledgeable judge on a show field can usually see the difference in these gear boxes and points will be deducted on your score sheet.
I have made this change on one of my '35's that is being built into a traditional hot rod and also plan to change my '35 5w coupe that will be a driver. I would never consider changing my Rouge Class fordor sedan Avatar car.