Best way to Install new cab support channel

[old Rusty C10]

I did one like this and i hope it helps.. you sand the edges of the new cab support mount so you can see the spot welds and then drill out the spot welds. using some wood on a floor jack or one of those large things you put under a lift you just take the weight off what is left of the floor. If possible mark the underside of the floor too as to where it was exactly sitting. then remove the bolt and body bushing and lay in the new mount lining it up with where the old one was at. I dont know how bad the floor is in yours but i was able to use some self tapping screws to line mine up. I didnt do a complete floor pan on mine i was able to cut out a section and did that next self tapping that in. I was also able to cut away most of the inner rocker then and made sure it was able to line up once i had it lined up i lowered the cab that little bit and i tacked in what i needed to tack . the idea is to cut out just enough without letting the cab get out of whack. also you want to make sure when cutting and welding you have plenty of eye protection and spark protection . I also did this using a lift