for bolting one up to a TH400 you will need a TH400/208 adaptor housing and the output shaft out of the same TH400. get the 32 spline input gear and the clocking ring from JB Conversions. their input shafts fit perfectly onto my TH400. plus the clocking ring will help you to lay the dana 300 almost perfectly flat between the fram rails.
you can run a 2.5" exhaust on the passenger side but the inner side of the exhuast pipe should be flatened a little to give a little more space between the dana 300 and the pipe.
i have mine installed in a 74 j10 with dana 44s front and rear. the driveshafts will bolt right up. the rear shaft will work fine if your running a 4" lift. the front will work fine if you are stock height. a little difference in height, the drive shafts will probably still work but you could run into something breaking if the suspension is fully compressed (rear) or fully extended (front) take the time to do some measuring.
i'll be installing a set of j20 axles in my j10 in the next week or two so i'll post up about the driveshafts then. i have yet to run mine so i have no feed back on that, yet.
03 GMC 2500HD - DD/toy hauler
06 Street Glide - Cruiser