Ok, BMS4 (show guest with only two posts since 2009), I apologize...
I've been out-voted that you are not just pulling our leg...??? To me this falls in the same category as explaining the difference between a door slammer and John Force Top Fuel Funny Car... but here is the answer...
a Super Stock car is a full STEEL bodied factory production car with stock type suspension in stock location, engine placement in stock location or sometimes with #1 spark plug even with upper ball joint, stock firewall in stock location, usually limited to 8" wide wheels and tires, stock type shocks, stock appearing body usually weighing in about 3200 lbs, stock type transmissions, stock type clutches and flywheels, stock type fuel pump and steering pump in stock location, usually limited to flat tappet cam 10 to 1 motor with headers and gasoline only. A good competitive Super Stock can be built for about $3000 to $6000.
a Modified is an ALUMINUM bodied highly refined piece of machinery, with almost nothing being stock, except the front frame stub, and very few restrictions. Everything else after that is lightweight fabricated, altered, modified and tweaked, resembling nothing even near and stock car. It has all aluminum body, fiberglass and/or plastic body pieces, engine set back behind cross-member, driver tub with no full floor or stock firewall, and they usually weigh in at about 2350 lbs. The aftermarket exotic engines cost upwards of $20,000.00 or more for USMTS full roller cam spec, to about $8,000.00 or more for IMCA flat tappet cam spec. They run either alcohol or very high octane race gas on special after market rear drive pumps for USMTS, or can run front Gilmer belt type drive pumps for IMCA. They have exotic 4 bar rear suspensions with numerous exotic components costing several hundreds of dollars, exotic shocks costing $300 to $500 each, direct drive two speed aluminum transmissions costing over $1300.00, quick change rears axles for USMTS or USRA, or Ford 9" floater for IMCA. Spec racing wheels and tires depending on what sanctioning body you attend with determine a level playing field. A good competitive Modified will set you back $35,000 to $50,000.00 or more, depending on which sanctioning series you run
These cars are very twitchy, unpredictable and have a nasty attitude beyond description.... extremely difficult "On the Edge". It takes years of seat time experience to be able to pilot one of these insane masterpieces... much like trying to pilot a top fuel funny car...
I hope that explains the difference clear enough, and I apologize again for questioning the integrity of your question... Jim