3.91 gears are great especially if you plan on hitting it with spray...with spray are you still thinking 11's? If thats the case its almost ridiculously easy to make it! I have a stock stroke 340 thats .040 over 10.1 TRW pistons for pump gas! stock rods and crank unported 308 heads with a 2.02 intalke valve in them .508 hyd cam runs great in traffic never overheats torker II single plane 750 hp holley 1/5/8 headers 3 inch exhaust 3000 stall and the EVIL KILLER 4.56 gear with 28 inch tall tires. its runs 11.90's at 110-112mph not even breathing hard...so take out the 4.56 put in a 3.91 lose what? 2-3 tenths? now runs 12.20's to 12.30's then throw in 150hp plate and now runs 11.30's...whats so tough about that? I run M/T ET Streets too. My engine is essentially a pile of used parts and it runs good