so my frame sucks...

I actually got under my truck today to have a look at things underneath, and everything is seriously rusted. There are patches of 1/4" plate down the sides of the rails, and a long strip of 3/8" on the bottom of the rails. There is also some tar crap sprayed all over the whole underbody, and it looks more like roofing tar. My brother wanted to start a stock floor on this thing, so this looks like the chance to take. So is there any good advise or things to watch out for when going with a firewall back frame?

Make sure you have solid strong unrusted frame to weld to.

Could it be POR 15 all on the bottom? But like uncle fester said make sure you get good welds on the frame. Get that rust off good.

nope its not POR 15 on the bottom, it smells bad and sticks to the grinder. But I might have to reinforce the frame at the front to weld the 2x3 to. I would use 2x3x.250 for the frame, right?

If you want a clean front clip to start a stock floor with hit me up. I have the forward section of the frame from my 92 sitting at my shop. Its cut off about 1/2 under the cab right now. You could cut it back to the firewall line and start construction from there. I am just wrapping up a stock floor for a mazda now that was done for the exact reason you describe, rot and lots of it. This one was rotting all the way to the centre line of the front x member. You will probably find yours is about the same when you really start getting into it. The frame section I have available is already bagged in the front as well. No bags but all the brackets are there along with the lower control arms. Hit me up if you're interested. PS 2x3x.188 is plenty strong to stock floor a mazda with.

Thanks for the offer, but I'm already running my CanDo arms on the front. And for some reason, the front isn't bad, its under the cab and back that is total garbage.

Just bodydropped my second truck first truck I used 2x3 by 1/8" wall this last one I used 2x3 by 3/16" wall. Loved it alot more. Basically a stock floor is not as bad as you think. You just have to have the nuts to take the truck apart totally and know how to get it back together. Label everything you are uncertain of or won't remember. Then cut the frame off in front of the firewall start with your new frame and plate the joint with something nice and strong such as 1/4" plate. Remember to take every possible measurement you can think of and also it helps to trace the frame out on the floor.

The crap on the bottom of the cab you are talking about is just factory undercoating. My cab is the same way. If ya have any questions or need some pictures to give you a better idea check out my profile or my thread under the "Under Construction" forumn named "Project: My Other Girlfriend"

that would be spray under coat. shit stinks like hell, gas or diesal should take ti right off. degreaser should to since its mainly diesal.

Or I could just throw the frame out Never understood the point of underspray when the whole frame rotted out underneath the coating

My only advice would be make sure everything is level, make sure the fridge is stocked with beer, and take lots of pics!