[TowerTalk] How to get another 10' in height? Add a mast or add another section?

[John W]

If I have a Rohn 45G tower of a given height, let's say 90', guyed per the Rohn book, and rotating from the base, hosting several side-mounted yagis for 10m (3), 15m (2), and 20m (1), and I want to add another antenna up at 100' (a small 40m 2L) that also rotates with the tower, which one is the safer/better installation:  adding a mast that protrudes 10' above the top of the tower (and for some distance down into the tower), or adding another 10' section and sidemounting the new antenna at the top of it?

This is a theoretical question, because I haven't built the tower yet, but I plan to.  

The first reply many might have is probably: Why don't you just build it to 100' per the book, instead of 90'?
The answer to that is that I don't have enough real estate to put guy anchors 80' from the base, I only have 72' available.

Also, I have not even seen hardware that allows adding a FIXED mast at the top of a Rohn tower - sort of like a thrust bearing that can't turn. How is this typically done?

Thanks & 73,

John
W2ID