Repairing frame with XJ parts

[Mudrig150]

I've been contemplating what to do on my truck for a while now, considering the condition of its floor and frame (or rather, the lack thereof). And I got to thinking... The frame rails seem to be the same width between my XJ and my MJ, and the only difference is the height of the rails, which doesn't really matter to me, as the sides of the rails aren't the issue, just the bottom corners.
So, I've decided to see if I can chop the frame rails and some other various pieces of steel out of the XJ body to transfer over to my truck. I mean, it's not like I can make it any worse than it already is. The XJ is basically worthless otherwise as it's got some questionable modifications, pretty much no good parts left on it, and a ton of rust in the back from fertilizer sitting in it. 
That would make my life a lot of easier, as the rust on the XJ's floor aren't nearly as bad.
My plan is to cut out as much as possible, then piece together what I can to get the truck's frame back to an acceptable state, then finally have someone cut out some brackets to remake the cab braces as they're completely rusted off of the floor.
My truck's frame on the driver's side, it's actually worse than it looks, the hole is actually about 3"x3" after poking at it, and it's getting scarily close to making the crossmember fall off:

Passenger side a tiny bit better, but not by much.
Vs the XJ frame:


Eventually I do plan on bracing the whole frame but right now I need to put my money into more important things like getting this truck actually driving.

[ghetdjc320]

The unibody frame area under the cab of the MJ is a bit tricky. The bed frame comes all the way up to that cab cross brace area and is essentially pressed into the unibody frame work. If your frame is otherwise straight, I’d rig up some 2x2 square tubing or even 1x1 and tack them onto the existing frame rails almost like crossmembers or a jig, then get the blaster/grinder/plasma torch out and see what it takes to get to clean metal. I can understand the priority is to get it running and driving but unfortunately there may be no great fix for the frame work that would allow for a cheap solution to get it driving. I’d start with the cheap stuff though just incase you decide that it’s just not salvageable (or just not something you want to take on). If you have decent fab skills though along with time and money, you can make it work. If this was my project and I was committed to restoring the frame, I’d brace it, media blast it (or use whatever means necessary to get to clean metal), cut out the damaged areas, weld in new metal, then add a full set of frame bracing. Lots of work though 

[Pete M]

fun fact, the individual sheetmetal frame pieces are different between an XJ and MJ.   probably close enough to make work, but it won't be an exact fit. 

[Mudrig150]

Close enough is all I need. I just need it better than it is, as you can see the frame flex on the driver's side when you bounce the suspension behind the front leaf spring mount, and the front link mount seems to already be deforming the frame rail at the front, where the XJ rail is solid.

I don't need it road drivable, just farm drivable for now, as I want to use it.

[pizzaman09]

I've learned with sheet metal work on vehicles is, close enough is easy to fix with a hammer and a welder and potentially a little extra plate material.  It's not like you are aiming for factory exactly the same.  

I fixed my driver's side floor with flat plate welded in, all welded with a mig welder that I didn't realize was out of gas.