I have a friend that modified his Suzuki sidekick, it's quite the beast. Anyways his rack on the back holds a 33inch McT mud bogger. The rack has been modified to make the tire clear his bumper when the door shuts... Anyways he's got it all welded together and all he did was raise the lugs... he never modified the actual door or mounting plates.
In short, I imagin that if the hinges and the rack can hold a tire that can't weight less than 100lbs... it'll probably hold your bikes n/p...
If you're really concerned about the strength of things you could probably fab something up that goes in place of your spare on the lugs and leave the tire in behind the back seats...
bigtire.JPG
That's one shot of the tire, I've got better ones on my home computor... (upload soon)
Hope that helps

-D-