Tranmission output shaft seal, Replacement is differnet than orig????? Options

[Michael J]

I'm on the final leg of putting my gearbox back together and all is going pretty much OK for a first-timer.
I popped out the old output shaft seal and find it has a nice lip to fit into the casing but the replacement does not have this lip. Pelican mentions this and I suppose it's OK but without the lip there is nothing to keep the seal from moving into the gearbox. I thought I better ask if this is right because I really don't want to have to pull things apart again.

Since I have your attention is there any advice for easily torqing the the 30mm nut on the end of the pinion shaft and the castle nut on the end of the drive shaft? I don't suppose a pipe wrench is a good idea. Didn't think so. I have the sockets but need to hold everything in place while I tighten them. Also there is very little room to get a socket on the castle nut.

Thanks.