Going to go with a 24" bar as you suggested. I decided to make the mill out of black iron that I have lying around from gas piping jobs. I have most of the design worked out but was wondering what you guys think for supporting the saw. I was thinking of some type of a platform off of the side of the mill that the power head can rest on. Also, BobL's post on mounting points on most bars was very helpful.
Just resting the saw on a platform is a recipe for a badly scratched up and eventually wrecked powerhead as the vibe will make the powerhead rattle against the platform. No amount of padding will work. The power head will somehow have to be bolted on to the platform to reduce the vibe effect. If you are doing that you might as well bolt the carriage to the bar bolts.
If the carriage is strong enough you won't need a platform for strength purposes. If it is not strong enough you shouldn't really be milling with it. Rail and Alaskan mills have their powerhead laying over on their side suspended all day every day just fine.
My Alaskan Mark III. as I've set it up, has a 21.5" throat using a 28" bar, but I left the dogs on.
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If jimdad can make a saw carriage he can do the nose and bar mods required to most of that 6.5" back.
If jimdad doesn't need the saw for other purposes, because wheels or skids serve no purpose for a simple carriage mill I would go with #8 on both the nose and the inboard bar connection.
One downside of a CS carriage mill is if you want to touch up the chain while the saw is on the mill (and I would recommend you do that ) you have to learn how to touch up cutters with the saw blade oriented in the horizontal position Of course it is possible top undo the bolts holding the bar/saw onto the carriage and completely remove the CS from the mill but I would keep that to a minimum. Firstly bolts wear out, the more often you do this the more often you will forget to tighten something. Fitting the CS to a carriage using #8 will also be fairly awkward. While it's a bit of a PITA I'd suggest learning to touch up with the bar held horizontal using a portable 12V electric grinder.
The other alternative is swapping chains.
While the chain is well exposed on the #8 design, unless the powerhead is supported in some way it is also a PITA to change chains while the powerhead is on the carriage on its side. This is where a little platform could come in VERY handy, but not permanently connected platform to the movable part of the carriage but at one end of the rails so you can sort of park the saw when its not running and the head is supported while you change the chain and mounting the CS to the mill. Under normal operation you should be able to keep the same chain for at least a whole day. At the end of the day I would at least suggest servicing and flipping the bar and then sharpening the chain off the carriage.
Here is another off the wall idea, what about making the whole carriage able to tip backwards so the saw is then upright - easier to mount the CS and sharpen the chain. I reckon that idea is a winner!
