Roger that. 1-1/4" square tubing fits perfectly inside BB ribs. That is what I used...then applied an overlap of cut down OEM rib to each section using a combination of welding and quarter inch steel rivets.
Interesting....
I had to use a hammer on a few of my 1-3/16" inserts. No way I could fit 1-1/4".
There is another build on here where the member used 1-3/16" that he found in a salvage yard for his BB raise.
I wonder if there were different hat channel profiles for different years or different factories?
Also, all of the exterior rivets I have removed on mine are 3/16" also.