I replaced all the hard and soft brake lines on a 49 3100 with the assembled truck on jack stands. Ordered the steel hard lines from one of the popular on-line distributors that advertise here and they fit perfectly. I reused the original junction blocks. A few leaked at 1st but a loosen and retighten corrected that. I got the soft lines, brake shoes and front wheel cyl cups at a fraction of the cost from a local FLAPS (O'Reilleys).
I had to order the new rear wheel cyl's & MS from on-line sources. Shop around as they vary in price and availability.....
Redoing the entire brake system on an assembled truck on ones back is not the ideal way to do it but it is doable. Those old HUCK brakes work pretty well when everything functions as designed.

Jack it up and put it on jack stands at all 4 corners, as high as can be safely done and wear eye protection. The hardest part for me was the front crossover line as I wanted to make sure it was fastened properly in the crossmember.

Take your time and good luck
Dave

1949 Chevy 1/2-Ton 3100
1949 GMC 1/2-ton 100
In the Stovebolt Gallery
More pix on Photobucket
Video of the 216 running
Video of a Winter Cruise