I'm just about ready to weld in the lower rails.  I don't have access to a "spot" welder, so I thought about doing "rosette" welds with a MIG.  Anybody do this or will it get too hot?  I might be able to get a friend of a friend to TIG weld them in, but the MIG is more readily available to me, and I'm doing quite a bit of bending these days!

I have done 10+ sets of lower rails by rosette welding them in place.  Just cool the receiver after each weld and you will be fine.

after positioning the rail, I clamp it with a vise grip, then drill an 1/8 th in hole through the receiver and rail, then open the receivers hole to a 1/4  inch, then weld it up, grind it flat and paint, works great on the ones I've done.

Quoted: after positioning the rail, I clamp it with a vise grip, then drill an 1/8 th in hole through the receiver and rail, then open the receivers hole to a 1/4  inch, then weld it up, grind it flat and paint, works great on the ones I've done.

Exactly!

Quoted: after positioning the rail, I clamp it with a vise grip, then drill an 1/8 th in hole through the receiver and rail, then open the receivers hole to a 1/4  inch, then weld it up, grind it flat and paint, works great on the ones I've done.

Why would you drill through the rail also?

Before I bought a spot welder, I used to rosette weld my rails in.
I drilled a 3/16" hole through the receiver, but left the rails intact.

I use a slightly different technique.

I take sharpie and draw the shape of the rail, then drill some 1/4" holes through the receiver, debur, then I have a mini C-clamp. I use the axis hole and center support and a "correct size" drill bit to space the rail. then rossetta weld. Also, what I did last time, was preheat the rail with a  torch, to almost red, then rossetta weld. no problems with weld not sticking to rail that way.

I had done a couple without preheating the rail, and the weld broke quite easily. when you preheat, the weld ends up being flatter, and with  8620 , you are supposed to preheat it for welding.   (the old flats that scott used to sell)  

edit: I preheat only the hole I am about to weld, then cool with a damp towel.