No professional welder will touch galvanized. Something to keep in mind...the fumes will kill you as jnelson mentioned. Look for the coating if you're buying scrap, it's easy to pick out...shiny matte-textured silver when everything else is dull grey, black, or rusty. Avoid it unless it's small pieces and you have a blasting tumbler or something...it's likely coated on the inside as well and you won't get it off.
The PBB is a GREAT place to read up on different processes (HREW vs DOM), different materials (1010 vs 1020 vs 4130 steels) and different fabrication techniques (GMAW vs GTAW, heat treatment). Discussions there border on religious, with more numbers than most people want to read.
Some interesting things I've picked up there, and from the engineering degree:
Sch40 steel pipe (vs Sch40 in plastic and other materials), HREW 1010 and 1020 DOM will deflect about the same amount for a given OD/wall thickness, even though one is ultimately stronger than the other.
You can make whatever you want from any material you choose as long as it's supported correctly...the advantage of using DOM 1020 vs HREW 1010 vs Sch40 steel pipe vs DOM 4130 is being able to decrease the wall thickness, so what you're building is lighter. Plenty of people have built practical, well-designed, tested, low-speed cages from Sch40...just like a poorly designed structure will fold regardless the material.
Wall thickness resists dents, diameter (actually profile...think of an I-beam) resists deflection.
If you put all that together, it means you can make a perfectly functional set of sliders, bumpers, cage, whatever, from Sch40 steel pipe, but it will be heavy. It also means that you may want to consider wall thicknesses and OD based on unsupported lengths and references such as your skid plates.
Keep an eye on how you attach things to the truck...support the load over as large an area as possible, over as structurally sound an area as you can manage. This doesn't take an engineering degree...it just means that if you can spread the load from top to bottom of the frame, and do it over a crossmember, it'll be more robust than welding a 1.5" piece of tube to the center of your 6" tall frame somewhere between two crossmembers.
If you choose to use materials which are thicker than your frame (I did), make sure the frame is adequately supported (I did this also).
Unless you have a particularly heavy vehicle, 0.120 wall makes a fine pair of sliders. My truck weighs 3T loaded, so I used well-supported 0.180 wall. Your T100 is of similar size to my Tundra, don't know about the GVW tho or anything else, but my sliders are 2x2x0.180 box for the main rail, 1.5x1.5x0.180 box for the ladder bars and 1-1/4" Sch40 for the kicker.
When building bumpers, bear in mind that bombproof construction will kill you and/or anyone you hit if it cannot deform to absorb an impact. Stick to 0.120 wall for bumpers unless you have an extremely good reason to do otherwise, for example if the part youre making isn't something that will take the brunt of an impact, or has a crumple zone behind it. For example...ARB bumpers are made from 1/8". Mine has a few mods here and there, like 3/16" plate replacing the sides where it was trimmed to provide more rigidity and protection. The sides will not take the brunt of a front impact, and the bumper is attached by a crumple bracket that will bend (hopefully before the frame) if the bumper is hit hard from the side. In retrospect I could have used 1/8" and a couple gussets and it would have been fine. My entire rear bumper is 1/8" except for the towing support and subframe, a couple small gussets supporting the hoop on the rear, and the shackle hangers.
-Sean
I geek out on that stuff!
