I'm in process on that job right now. I have some 1/8" diamond plate and find that to be a very solid floor. It will definitely be heavier than required, but I don't mind. Be prepared to repair the floor supports as well.
I couldn't have planned this job if I'd tried so I just dove in and started ripping out everything that was rotted out, then I wire-wheeled the remainder. Fabbed up supports for the floor, replaced parts of rotted posts, and finally floorpans. I think that you will find that any of the supports can be repaired with c-channel and angle iron. Nothing heavier than 1/8" stock is required. Get lots of saw blades and grinding wheels!
Go carefully when welding back in and don't forget to tack on the belt and seat mounting harware. I ALMOST did.
I am really liking the way the diamond plate is looking as I put it back in, very Bada$$. The only problem is that stock carpet is not going to conform to the new contours, and I'll lose teh heat and noise retarding properties in that regard. I'll likely go the Herculiner route, but still intend to insulate the tranny hump and fab something up to cover the insulation since that is likely to get hot in the summer.
John