Here is a lame presentation of how I setup up the first cut on a log.
http://www.arboristsite.com/showthread.php?t=73696
The only limitation to the jig is the length of the 2x4. I started out using a stihl 044 and new that it was underpowered so I moved onto a 066. The only wood close to what you are cutting that I have cut is cottonwood. It was really easy for the 066. I am used to cutting oak, cherry and walnut so the 066 is definitely needed. 20" plus usually.
Goodluck and be in shape before you start sawing. It will beat you down if you are not.
Chris
I made up brackets from 2" aluminum angle to do the same job as your square tubing. While you use clamps, I use drywall screws to hold the 2x4s to the end brackets, and the brackets to the log. My 2x4s are 10' so I cut some "rungs" to stabilize the 2x4s. Once the ends are fastened, the rungs can be positioned to touch the top of the log and then screwed to the 2x4s so as to prevent flexing up and down.
10' 2x4s are about perfect for 8' lumber or beams. 12" lead in/out makes for nicer beginning and ends of the cuts. My problem is that my truck has a 6-3/4' bed so long stuff has to stick out the back. For longer stuff, I join 2x4s together to make whatever length I need.
If you decided to try the 2x4s for guide rails, be sure to eyeball them and make sure you get straight ones! The mill will follow the contours of the rails.
I made my first cuts with a 272xp and 28" bar. This setup allowed me to adjust for 21.5" width maximum after the bark dogs were removed from the saw. I expected that Hemlock would be easy to mill, but the 272 seemed a bit slow despite new chain(round chisel). I swapped the 066 onto the rig later with similar chain, and it cut at least 3 times as fast! Displacement makes a BIG difference.
I also have a Haddon Lumbermaker. I finally got to try it a couple of weeks back. The piece of hemlock that I had remaining was 10" or so in diameter and 10' long. I made the top and bottom cuts with the Alaskan mill, and then just screwed a 2x4 to the top for the Haddon. I mounted it to the 18" .325 bar on my Husqvarna 353 (53cc 3.3hp) and proceeded to cut the sides. It wasn't particularly slow, but didn't yield as smooth and uniform of finish as the Alaskan. The finished 6x6 was only going to be used for a mailbox it didn't matter. With this type of equipment, the 029 should be fine. As for finish, a planer will clean up the rough stuff anyway.