after this step is it pretty straight forward or is there anthing else i need to know...
I used a large bar jammed against the chassis and between the studs on the companion flange to hold it, and a 3/4 drive breaker bar with about a metre extension to loosen the nut, and even then it wasn't easy. The studs are long enough to jam a 3/4" steel bar in clear of the threads. It's somewhat worse than getting axle nuts off cars when replacing CV joints etc.
Then a 3-jaw puller to get the shaft out as above.
Next problem I had was that the old seal was very tight, I couldn't lever it out, and also tried drilling it, screwing in self-tappers and pulling it out that way, but that also failed. In the end I got it out by grinding the heads of some small bolts so they'd slip between the seal and the output shaft, then in behind the seal, put the nuts on in the right place and gradually levered them with a claw, with a bit of hardwood protecting the end of the shaft. There's possibly a bearing puller that would work on this seal, but I didn't have one.
You'll drop a bit of oil when the companion shaft comes out, then a bit more that's sitting behind the seal, but the bottom of the seal sits well above the top filler plug on the transfer case, so there's no need to drain the transfer case.
I used a Nissan OEM seal. The price was totally outrageous. The companion shaft surface wasn't scored, I guess the original seal might have been stuffed if at some stage the drum got hot from binding or driving with the park brake on, or it was just plain old - 1988 .
My seal was leaking very little with no drops on the driveway, but enough over the years to degrade the bonding between brake shoes and linings, even though the brake seemed to work just fine. The linings slid clean off the shoes when I parked on a steep uphill gradient, fortunately just before I got out of the truck. Potentially very dangerous - I suggest to wipe your finger around when bungs are removed from the drum for brake adjustment, and if there's any oil traces, remove the drum and check it out and replace the seal and shoes if needed. Likely scenario for it to have failed would have been when launching my boat (2 1/2 tonnes) and I was behind it undoing shackles etc.