Hello Wade, I hope that you can get enough information from the photos to get yourself to a successful changeover. You'll note where I parted off the front portion of the old aluminum endpiece, that dimension was based on using the length of the particular 4" brass nipple that I was able to purchase at the local plumbing supply. So, you might have some slight dimensional differences due to your particular supplier and the overall length of the 4" nipple that you come up with. Any differences there should be able to be addressed when you cut the silcon-bronze liner to fit, if you even have to.
There may be one additional benefit to doing this as well, if you are a user of "Centerfire" consumables, nozzles, and diffusers, they can be just as easily adapted onto this set-up as the internal threads for the diffusers are the same. So you have other options than just the "Fast tips", as a matter of fact, Bernard diffusers could be screwed on too, don't know about Twecos though. Good luck and best regards, Allan