I believe you will find that the dimensions are as delivered, but some have had problems with the brackets cracking since the frame sits in the middle of the mounting face and the main force should be aligned with the side of the bracket that should line up along the outside of the frame.
The inside dimension of the frame is (on my 16' Sacmp) 45" and the outside is 48".
The frame is made of 3" x 1 1/2 " square tubing stacked on top of one another.
I ordered my new Flexiride axles with 48" outside mounting bracket dimension so that a plate welded on the outside of the frame can overlay the side of the axle
bracket and be bolted in after alignment and the bolts "killed".
My 1986 16" had face to face dimension of 65" which I retained. The new axle
will have 5 on 4 1/2" bolt pattern instead of the 4 bolt.
If anyone needs some wheels and tires
for the 4 bolt hubs I have three extra with fairly new automotive type tires
. I also have an old axle
(2500 lbs) with 4 bolt brake hubs and brakes
complete and probably never used since the trailer had a 4 pin connector.