Mike, done this on 3 different airheads...very simple. Just did on my 78 100/7 this past winter. Also did on 82 RS both front and rear crossovers.
Here are dimensions I have off the 78 for a front crossover just completed. Used the same crossover pipe length (chromed 1 1/4" sink drian pipe extention) for the RS both front and rear as recall.
front crossover pipe length 10"
distance between front crossover at header pipes - 12" +/- 1/8"
Same for rear crossover on my 82 RS
Cut the chrome 1 1/4" drain pipe lngth to 10 - 11" (mine is 10" on the 78 but there's additional room to go to up to 11" if you want)
I put 2 dremel cuts at each end off the pipe about 1/2" deep to allow flexibility in the pipe ends to slide over the crossover pipes coming off the headers...
Slipped the 1 1/4" "new crossover pipe on and used 2 airplane clamps to tightened down.
Works like a charm
I loosened off exhaust nut only on right hand side (as well as exhaust hanger in rear) so could turn header pipe out aenough to get old rusted out crossover off and new one on. Slip new crossover on one side then line up with other and swing back (rotate) loosened header pipe and lock everything down....very easy.
Here's some pix of install on my 78 R100/7 (sorry for the crap cell phone pix)