Welcome to the site! If it's just for parades and an occasional ride I think I would just leave it and add the wrecker to it. If you want to take it up to hwy. speeds, then you want to upgrade to discs and change the rear gears. If you want a workhorse sounds like the P30. But, it sounds like you are leaning towards the '77.
I've seen some guys who chop the frame of the '55 and fit it directly on a later frame. I believe that the tires will stick out on either chassis. Look at the sticky note at the top of the "Big Bolts" forum on disc brakes. Some of this is mentioned in there. Do some searches and you should be able to come up with it.
