Good to hear it was an easy fix.
I assume the two of you are working on the same project.
Don't think you will have any issues with tech on the FJ box.
As to the question of running 15" wheels; You will change your final gearing ratio using the larger wheels and tires. Also, ride height and handling will be affected.
I bought some of the old BFG's for my car. They can be had pretty cheaply. There are a couple of ads on the site for both wheels and tires. I've seen tires (BFG) anywhere from free to $20-$25 each. Could save a few dollars on shipping if someone in your area has some. Since neither of you have your profiles filled out, I do not know what area you live.
To meet the SCCA autocross rules, there are only a couple of tires you can use and still meet SCCA rules for Legends Cars and run in the F/M class. Otherwise, you will end up running against modified Corvettes, Mustangs, Camaros and the likes.
Message me. Would like to trade some info on autocross.
Steve