If I wanted to lengthen my suspension from 121 to 136 and also put on 8 inch ilders on the rear, would I need a set of rail extensions that were a different length? If so what length would they need to be?

You need to specify that you are looking for an extension kit set up for the big wheels. Axles are located differently depending on intended wheel size - try: tracksusa.com

Haven't tried them yet but they are part of my plan. Appear to be very high quality, reasonably priced, come both ways(w/wo big wheels). They also use hyfax all the way back to the rear wheel. Some other don't.

AL