I use a ProMac 700 for milling -- it works well enough for the bit I do, but it's certainly got it's limations -- your saw will do quite a bit better no doubt. I've got a chainsaw mill with a track based on the Procut Portable sawmills plans, with some minor adaptations.
Mostly I haven't done any real modifications to the saw itself for the purposes of using with the mill, but I do the following when I use it:
- Remove the brake arm -- I actually have a 2nd clutch cover that's not 100% but works well for this purpose.
- I have a gas cap that I have drilled to fit a piece of fuel line into. I was going to use this to feed fuel from an auxiliary tank, but I ended up just using it for an upwardly extended vent for the tank (I've got one way valve and piece of sponge for a filter on the end) to keep gas from seeping out of the tank when it laying on it's side all the time. The whole saw carriage is pretty easy to tip on it's side when re-fueling, so I don't find I need an auxiliary tank -- If I was doing a lot of milling, I'd probably add the tank.
- I have scooter velocity stack I got off ebay and modified (made the inner diameter a little larger) and a unifilter sock filter with nylon stocking pre-filter that I use -- I just leave the cover off. I find the cavity above stock fuel filter setup is prone to filling up with sawdust when you're always running the saw on it's side.
- And then I retune the carb while I'm running it.
I don't know how applicable these minor mods would be to other Mccullochs and othter types of mills.