Pumah 25,
I did exactly this .30/30 to .38/55 conversion a few years ago. Gun parts corp sold an entire "kit" the longer octagon barrel, the forend, half magazine tube with spring, and forend cap with screws.
I found a '70's winchester at a pawn shop that had a rusted and pitted barrel (just neglect) and put the parts on the receiver. It worked out real good, nice smooth function and a darned good shooter too.
look at Gunparts /Numrich arms and see if they still have the kits, having all the parts to just assemle made it easy.