LBP to SBP wheel adaptor?

I am not positive, but I think you will have a problem with the brakes. SBP 7 1/4 rear had 9 inch brakes, and LBP 7 1/4 (I think all, but know most) had 10 inch brakes. All the brake parts are different. Even the backing plates, so you can't swap the brakes over. The early 9 inch brake 7 1/4 had a 4 bolt flange to mount the backing plate, and the late 10 inch brake 7 1/4 had a 5 bolt flange. Even the axle bearings are different. The center hole in the drum is larger with the 10 in brakes, to match the axle.
I think your best bet would be to swap to disc brakes on the front, and swap your wheels for large bolt pattern. Most people have the reverse problem, they put LBP discs up front and then can't find a LBP rear.