I could be wrong, but I don't think Mark meant 13" was too big to fit. He meant anything over 10" was going to be a challenge to make fit. Like I said, you aren't the first one to run a core this large. Your time would be better spent modifying the setup you have, rather than starting over from square one with a smaller core IMO. Either way, you will have to modify piping and brackets to make the setup fit, so why not stick with what you have since it is already hanging on the car. I assure you, the core you have will fit, but again, it is going to take some work. The core on 1813 was 12" tall, so you have the space required.
I can already tell you the bracket you have mounted to the top of the core is robbing you of a few inches of clearance at the bottom. That bracket is locating the core too low from the get go. That's where I would start modifying; not trimming the bumper cover. Remove that top bracket and position the core where in needs to be, then fit the bumper. Once it's in a position that works with the bumper, build another bracket, or modify the one you have to properly locate the core. You may need to modify the piping as well, which I wouldn't get discouraged about.
Or, buy a smaller core. Either way, you are still going to spend a fair amount of time getting it to fit right. Why don't you go ahead and ship it to me, and I'll show you how to make it fit? /ubbthreads/images/graemlins/wink.gif