Hi Craig
Welcome to the site.
When you get a moment throw up a few photos of your '54. Will be great to see the project.
Skipping around is a great practice, though I've found that using compressed air to cool the weld seems to make it harder and the area more brittle when working the sheet metal after. While others may disagree with this, I've found from experience that letting the tack/weld air cool till I can put my hand on it then move onto the next spot works quite well. It may seem slow and tedious, but in the end the result speaks for itself. So basically, tack the panel until all the tacks are about 1" apart, then move around the panel welding it up 1" at a time, grind the proud off(excess material created by the addition of wire) till it's just above the surrounding metal, planish and repeat... Running the machine hot(but not too hot) also helps in reducing the amount of time it takes to fuse the metal and therefore keeps your HAZ area smaller. Practice on some similar gauge metal to get a feel for how it'll react and what settings work the best.
Here is a link that you may find helpful for Mig Welding.
http://allmetalshaping.com/showthread.php?t=3487
A lot of guys out there TIG or Gas weld their metal. But if a MIG is all you have then that's fine too. Just takes a little more work to finish the metal.
Hope this helps.