I still dont see how inserting anything in there is going to help, unless you do like someone mentioned and make L shaped pieces and bridge the connection point. Or even cut two L shaped pieces and weld to the outsides. With only 43" forks, there isnt going to be much bending at all up until failure point that will occur at the heel. Adding a piece inside wont do anything to strengthen the heel, thus would still fail with the same amount of weight, and maybe a tad less deflection along the length.
Unless I am not understanding what you plan to do. I envision a piece of barstock 43" long welded inside the tube in the middle vertically? Basically making it look like a pair of 2x2 tubes looking at it from the end?
Like I said, I think they are going to be strong enough as is, but you never did mention what this other tractor is? Adding what you have planned if I am picturing it correctly wont help with strength.