It seems there are no easy answers and it comes down to a labour of love vs cost and time and how long do you plan on owning or getting $ for the truck once finished. As Chris Wilcox says go with 3/16th plate. that is the frame thickness not what the rust has reduced to, so one can see you lost 1/16th as you came up with 1/8th plate.
If you have any welding skill draw up what you need in steel and have a local steel shop bend up the channels make sure they fit together, a round hole that Toyota has designed and is in the inside frame was designed not to collapse on it self. When you look at the 70 series frames they are mostly c channel without holes, maybe toyota found out the frames filled with earth, mud and dirt ?
As for the wheel arches if the shop cannot bend them then you will have to cut and weld the sections your self and then fit the finished channels together. For welding do stitch welds, each weld being 1 inch to 2 inches long and space them out, if you do a continuous weld then you might as well have a box frame and end up with other issues if you do : warpage from heat bending, will not flex as well as original etc...
If you go this route then you must not leave it raw, my first choice is hot dip galvanizing and hitting it with some undercoat.
If you like the idea of the suspension kit you might factor that in while your at it or factor in a nice rear receiver hitch.
Also : check the other forums where others have used 60 and 80 series frames for their 45 rebuild.
Re : Kurt : ''one for sale in B.C. '' That may be the easiest, cheapest and less time !
Either way if you think you are having sleepless nights pondering the options that is just the beginning but in the end it is worth it.
I did a rebuild on my 45 and hot dipped it and it came out very well.