I've never needed to modify length, but the u joints are different. You'll need a conversion u joint available from any parts store. It adapts the two different era parts together by having two different sized dimensions.
I'm currently running the opposite combination in my 78 truck. A driveshaft out of an 87 model truck which had a 10 bolt rearend with the 78 model 12 bolt rearend. The conversion u joint makes it all work.
By the way, a non overdrive trans, 2.73 gears and 28 inch tall tires is an outstanding combination for highway cruising. I daily drove that exact combo for a lot of years in my 83 stepside. It made my 40 mile freeway commute a pleasure.