Major bummer! Good idea, the full roll cage will be a must, also, doesn't the missing cab provide some structural integrity? Would have to come up with a way to re-inforce the remaining cab.
Keep in mind that it depends on what year. 87-96 XJ are generally direct bolt on. 97+ require some modification but can fit and are a nice upgrade, especially the header.
Thanks for the pics and future write up. So that seal puller just hooks one side of the seal and pulls it out? Did it work good or could a flathead do the same job?
If your putting it on your '86, all '87 and above need some modding. It has been done by several folks on here, you can check it out in the build threads.
Also, check the wire that runs from the battery/Alt to the distributor. It runs close to the block and I found mine had a bare spot and was shorting out.