You will want some flex in the frame, so boxing the whole length would not be good. I added some plate to my '37 where the motor mounts were welded in and at the back where the extra shock absorber crossmember was added. I probably went 6" for and aft of the mounts with same thickness flat plate.

As for the rear fender braces, mine were long gone by the time I got the truck. I have fiberglass rear fenders, so all I did was weld some 1/4" round stock to a flat plate and mounted it to the frame rail just past the tire. I used what I thought was stock mounting holes. Out at the fender I had a second plate welded to the round stock. I bent them down to meet the fender where I thought they would do the most good. I had to glue the plates to the fenders so no holes would be needed to be drilled.