The timing changes around when it's at idle? You mean, you put a timing light on it and the timing keeps changing? If that's the case, I would say that your distributor is toast and any mechanic that saw that your timing kept changing at idle and pointed at your carb needs to be strung up.

As for replacing the distributor, it isn't difficult. I did it in the parking lot of the parts store. On the 300, there is ONE bolt holding the distributor in. Take the cap off, have your new distributor right next to the old one and light the internals and externals up to eachother, take the old one out and switch the plastic casing and shaft from the old one to the new one taking care not to turn anything, and pop the new one in and bolt it down. Make sure to time it when you're done.

IF you mess up the settings on the distributor and need to redo it from scratch, that's not that difficult either if you make sure to follow ALL the steps in your repair manual. There's an alignment inside and every 1/4" is like 7* timing off, so make sure it's accurate. I think you set your number one cylinder to TDC, line up the distributor how it says to do it, and slide it back in.

Either way, it's a piece of cake. The only difficulties you might have is getting the teeth to slide all the way in and keeping it aligned at the same time. That just takes some finagling.