floor chanel replacement

Don't weld the channels completely to the floor to avoid warping the floor ( Like I did )

I would put anti crush sleeves where you plan to put bolt through to attach to tub.

When I installed new hat channels, I had good luck using a flanger/hole punch tool from HF. Worked well for me doing plug-welds.

X2 what Russ suggests.  It's a good idea to use the frame to align the channels so they mount up right.  All assemblies are a bit different, but  I bolted the channel pieces to the frame, then tacked them together as a complete assembly.  I completed the channel welding, then bolted them back on the frame.  I essentially rebuilt the tub on top of the channels while they were bolted to the frame.  In my case, this assured the tub would mount aligned and straight on the frame.  It is a lot more steps to do it this way, but I didn't want to take the chance that it wouldn't fit back tougher correctly.

Fitzie's Fabrications youtube on welding sheet meta: https://www.youtube.com/watch?v=qScAlxb34UA

I recently did this, I used the hat channels from Classic Enterprises.  Good quality product, they had the mounting holes drilled and crush spacers already welded in place.  So I lined up the fwd mounting holes to the firewall gussets and the bolt that goes threw the bracket by the steps.  It worked out pretty well.  I test fit this weekend and only the rear floor riser mounts were off a little all the hat channels bolts were good.  I also used the Harbor freight punch tool, but had to return it twice.  I would get about 10-15 holes punched and it would die.  Had correct air press and oiled it, the flange side is about useless.  Good luck.