replacing 67 sheetmetal / rails/ etc... need advice

Well, MIG is something very easy to learn, but that's only if you've got a good teacher. I think most people get the wrong idea that you can just pick up a MIG welder and start going at it and all will be fine and dandy. MIG has it's complexity. Don't get me wrong, it's the easiest welding you can do, but you have to know how to set wire speed, amperage, flux or gas, etc.

You can arc weld it, that's not a problem at all. I mean, chances are it was put together originally with an arc welder(MIG didn't get big till the 80's). But I don't know if a little high school experience will cut it. Also, you have to know how to cut the old panels out and line up the new ones.

Here's my advice from both a welder and a Mustang restorers point of view:
Take it to a credible shop, there's still plenty of other things you can do to the car(suspension, interior, engine, etc.). I am also a person that wants to do as much as possible to my Mustang, and I have, that's why I've owned mine for 3 1/2 years and it's still not done. But I know my limits(or I try to), and so I've had the bodywork and paint done by someone else, because I want it done right. I mean, there's no reason to give your hand at it not knowing how it will turn out(unless you have plenty of time and money) when you can send it to someone else you know will do it right.

Replacing floor or body panels is not that hard. Nor is using a Mig welder, provided you use a decent MIG.

A cheap welder will have a undersized transformer which will heat up and not evenly deliver the current. The arc quality will change as the transformer heats up. With these welders you need to allow a large amount of cooling time for the transformer. A welder is rated by the stated "duty cycle". The duty cycle is the percentage of time that you can operate vs just have the transformer cooling fan runninng. A cheap welder has a 10% duty cycle, while a decent one has 30%. My Miller Challenger 172, a nice $600 MIG with 30% duty cycle never gives me a problem with the arc. With a decent welder you mainly need to pay attention to the wire feed rate and the current (heat). It's easier to weld thicker material than sheet. With thin materials you tend to either burn through or have poor penetration. Most welders don't deliver wire real well at the slowest speed. But it is not that hard to learn.

Changing frame rails involves the chassis geometry (alignment). Just replacing the rails to the same position as the old ones may not be good enough, since the old ones may have moved within the old rusted mount points. Careful attention must be given to frame alignment specs.

I would tend to do the work myself, not because it may be cheaper, but because I wouldn't assume that a shop willing to do the job would do the job right. I think that in my area (Austin, TX) any shop that has the equipment and trained personnel will not want to take on the liability of welding up a rusted vehicle.

i've had 4 "proffessional" body and welding guys do my car in the last 10 years and wasn't happy with any of their work. either i was expected to do most of it anyways or they had no idea how the lines on my car were supposed to go and it takes alot of work to fix what others mess up. i agree doing it yourself seems to be a good way to go especially if u'r willing to learn and make a couple of mistakes lol. and getting a half decent mig will make things alot easier. i have a crappy welder and 1/4 of my welds have been ground off or stopped halfway thru to try to get it right hehe. i was told i'd never be able to do my car til a guy at a swap meet said "go for it, early mustangs are the easiest things to work on its like a big lego kit" hehe. u certainly get satisfaction from doing it right and knowing u did it yourself. i think i'm even going to try the paint myself my buddy who owns the body supply says that its not that hard, once u get the rythm. i'm sure it really helps to have a guy who knows what hes doin tell u how to, like the mixing and time between coats and stuff. i'm sure u'll have a blast and it'll turn out great. jsut get the shop manuals to measure and align the frames and things. and u'r second car will turn out even better than u'r first lol. good luck