I have a Jago Geep body in my (long suffering) mum's front garden right now, waiting for me to get around to making the time to do the swap.
The Geep body is designed for an 80" wheelbase chassis - the same as the SJ. Not surprising when you think that the SJ was designed to be as close to a Willys spec vehicle as possible, and the Geep is (roughly) a replica of a Willys Jeep. The front wings are made such that you can fit anything from about a 75" to a 90" wheelbase chassis under it (just line up the body with the rear arches, which are exactly the right size for 31" tyres).
If you swap the body onto an SJ, try not to modify the chassis. You are going to have to register the vehicle with VOSA as a "radically altered vehicle".
If you keep the original engine, chassis, axle, drivetrain, steering - you should sail through the inspection and they'll update the V5c to say "Jago Geep" or something similar (they often make you choose a name) and usually you keep the reg. number of the original vehicle.
Whatever you do, ignore the guys on the Jago Owners club who tell you to modify the chassis to match the body. Yes, it's easier than the route I want to take (find an SJ body, make a mould of it's underside, then use that mould to make a new GRP floorpan, fibreglass that into the body, then bolt it all together).
The reason I say ignore them is that a radically altered chassis (anything other than bolting parts on to existing bolt holes, welding in a new crossmember, or boxing in a 3 sided chassis rail) is going to make the car illegal until it's been through an IVA test (which costs hundreds and is *HARD* to pass).
Make the floor fit the chassis. That way the VOSA people can't whinge at you.
The official PDF file from VOSA explaining the rules on this:http://www.dft.gov.uk/dvla/forms/~/medi ... inf26.ashx
If you have questions, I'll try to answer them. I've looked into this fairly extensively. I'm no guru, by any means, but I'll try to at least point you in the direction of an official VOSA or DVLA document explaining things.
Here's an SJ Geep that had the body cut off, the floorpan was kept, and then the Geep body was "persuaded" to fit:http://retrorides.proboards.com/index.c ... =1#1331218
I may still decide to do that if I can't be bothered to make a GRP floorpan (which is a lot of work, but would eliminate rot problems and be lighter). My tintop is a bit rotten, and it's been mangled in a few offroad "incidents" in which it didn't stay entirely vertical...