I have had both issues in the past. The loose fit on the transmission is usually caused by taking out too much brass inside the tube when milling the ends. First try to be careful not the score up the inside of the tube. You could also coat the inside of the tube with CA and let it dry before pushing it over the tranmission. This will give a little tighter fit.
I have also seen where someone "peaned" around the tranmission with a punch of some sort. The row of little dents caused a tighter fit in the brass tube.
The finial/clip assembly issue, I sovled, by using an arbor press for assembly. The extra power and control, makes it much easier to press it together even for the stuborn ones.
I also saw in a book, how someone tunred a peg with a hole in the end. The hole is just big enough for the end of the finial to slip into. The O.D. of the peg is a little smaller than the diameter of a SL. That peg is then used to pre-assemble the finial and clip. That way you don't have to try to keep the finial straight and the clip lined up at the same time.
I hope that is as clear as mud [
)] maybe someone else can word it better than I.