yes, spring pin (roll pin) has to come out.
Don't use a 'drift', try using a bolt ! If it's a 1/4" roll pin, use a 1/4" bolt, cut the threads off and lighlty make it smaller in diameter. The 'trick' is for it to just fit inside the hole. Hold it with pliers THEN whack with hammer. A good, stout WHACK. The 'idea' is the shock will break up the rust.
if that fails, try drilling the roll pin out, again if 1/4" ,use GOOD, new 1/4" bit. Now it may 'grab' in the spiral of the rollpin, depends upon design of the pin. Pins are 'hardened' but they can be drilled.
Once out, use a few big open ended wrenches as spacers between gear box and Ujoint, insert pickle fork between last wrench and ujoint, see what a few whacks do.....helps to have trained octopus to hold the wrenches.
as always LOTS of oil, heat and patience will help !