Nantucket,
Mr. Fix it is correct on the method to determine what octane you are using. 93 + 112 = 102.5.
Boy,
I sure hope you do not plan on running this engine on the street? For one thing the cost would be phenomenal! It won't take too many long cruises until the only thing you will find in the bottom of your pockets will be lint!
427 S/O,
I have never tried home brews, but it appears that there is a little more to it than what you mentioned. The race gas as an additive is a whole lot cheaper and safer.
http://www.vtr.org/maintain/gasoline-octane.html
Ps. The prices for Tolene are extremely understated.
Bud