I have done a few full floor pans and body on frame like yours is the easiest. Installing a complete donor assembly with rockers attached may sound good in theory, but not in reality, in my opinion. Once you start cutting the old floor out, you’ll realize how many nooks and crannies you can’t get to but need to for welding. The donor floor assembly fit the body it came off of, but probably won’t fit your body due to dimensional variances during the original factory assembly. The rockers need to line up perfectly with the quarters and cowl, even an 1/8” wider or narrower isn’t going to work. I ordered a repop floor assembly for a Mustang recently, when I went to fit it, it was a ½” to ¾” to wide, I’m not going to stretch the body to make it fit like you see on the internet, so I cut the floor assembly and narrowed it because I want the doors and other parts to line up as original.
Go ahead and search for a donor, if it’s in good shape it will have usable pieces but I wouldn’t attempt installing it in one piece. You'll need at least 4 people to move it in and out multiple times.
If the donor is really clean, I would separate the pieces from it and reuse them individually. I would build the structure (rocker panels and braces) separately inside the body shell then lay the floor on top. The rockers weld to the body shell, the braces weld to the rockers, the floor skin welds to the braces and rockers. I would buy new inner and outer repop rockers and new floor braces to ensure you have good metal to weld. Pretty sure I’ve seen Safari repop rockers. I’m not sure if Chevy braces are the same, but should be close enough to modify unless you are a purist where every crease and bead must match as original. Once you have the rockers and braces fitted to the body, that is your main structure and you can do a nice job welding it all in place and it will be strong. Very difficult to align and weld a used assembly securely to the body shell mainly due to the condition of the old steel in both structures. Everything hangs from the rocker panels so your inner body shell needs to be sound on both ends where the rockers mount, or you’ll need to fabricate new plates to weld the new rockers.
I have done body on frame floors with the body sitting on the frame when the floor braces don’t need changing and I have done them on a rotisserie when new braces are needed or it’s a unibody. For the rotisserie I build a inner tube structure (1”x1”x3/16”) to support the body from twisting and to keep the body width from changing (I mentioned the Mustang width above). I also weld gages to the structure for floor pan height or other fitments such as rocker height that affect driveline and exhaust, references that I’ll lose once the old floor is gone. You don’t want the pedals to feel weird when you drive or the seat to be to low or high.
The Mustang below is a basket case that belongs to a foundation, the floor was shot, torque boxes non-existent, but the previous shop put new quarters over rust and seam sealer. They welded the front on crooked, the wheelbase is 1 ½” longer on the left side. Once the floor is in the quarters will come off and the front end will come off to make place for a Mustang II suspension kit. It will be a resto-mod and auctioned off. The other car is a Chevelle which has your style floor. The rockers and braces are perfect, so it's just a matter of replacing the floor skin with the body on the frame. Cutting it a little bit at a time.
