Look at the photo series "details, details" to see the start of welding up the seam and the finished work on Snakebit. Personally I would lay a length of 1/6" tig welding rod in the seam so as to not need as much heat and filler wire, especially if welding with MIG. Note how they are doing a seires of tack welds a distance apart rather than a continuous bead. They may look like a continuous bead in the picture, but they are a series of short overlapping tacks.
Wheels Of Dreams Project - Follow The Build

I highly recommend using 0.023 ESAB Easy Grind wire for MIG. See my welding tutorial:
https://www.ford-trucks.com/forums/1...-practice.html