Hi,
There's a guy near red Oak IA who builds frames like that style. but he does the grind it flush thing...
Your's Kinda looks like octopus legs, pretty knarly looking.
I noticed on the welds the wire speed is kinda high for the amps your using.
If you made these welds with a stick welder with that lack of penetration you'd have hella slag inclusions.\
Without burning through you need to gouge into the base metal the full width of the weld. when the wire fills the gouge back in, if you have proper penetration, the bead will blend seamlessly and smoothly into the base metal like when 2 water drops come together. you have some areas where that is happening but most of the bead is lopping over at the sides looking like water sitting on a good wax job... which is evidence some of the weld is properly part of the base metal and some of the weld is just metal caulking.
The difference is if the weld bead and the base metal become one like they grew together or if they end up just being stuck together.
It's also possible your feeding the wire into the weld the same direction you'd do with a stick welder. with mig if you push..angle the wire in the direction you welding, it will penetrate. the arc runs in advance of the wire throwing the heat into the base...
if you pull...it will heat the wire faster and pile up on you without burning in as good.
I think stick acts the opposite because of the flux...when you pull with the stick it burns in on the naked side of the rod and the arc blow piles the slag on top of the weld
Now the opposite of that is if your using too much heat and not enough wire speed you will get the good penetration but will end up with an erosion furrow along each side of the weld which will be where a crack forms.
If they go through all the trouble for the holes, why not leave them open?
I haven't seen one done without putting the pipe in... do they crack?
the pipe is adding more weight than was removed by drilling the hole, I always wondered if you could put in the holes and leave them.
If your using 2x4 box or channel, the 4" side web is stressed about 1" from each edge and decreases inward. the rest of the side web in the middle...carries little or no load, it just holds the top and bottom stringer apart.
so with 2x4, 2" holes down the center shouldn't weaken the rail much if at all.
I recall from some engineering research I did into splicing semi truck frames that with any hole drilled into a frame... simply a bolt with washers on either side of the frame and torqued down will restore over 80% of the strength lost by drilling the hole. With the truck frames they do not advise drilling into the side within the thickness of the top or bottom web. they require all holes to be drilled within the center 2/3 of the frame and no holes to be drilled at all in the top or bottom...
This of course is with high tensile C channel frames.
P.S...
I spent a year working as a MGAW welder building these: