I'm no QC expert but there are different sizes. Most common is the champ style - those are what you see on ebay. They are larger than the traditional hot rod type that uses original ford V8 bells off a banjo rear.
I believe the original quickchanges were ford banjo rears flipped over so that the pinion was now facing the rear. A new pinion was added low to the front with a cross shaft out the back, on this crossshaft was another gear added and a gear added to the pinion, to change gear ratios they'd just change the added gears from the jackshaft to the pinion - hence quickchange.
Companies like Winters make street quickchange rears that have the right stuff inside, they're pricey but sweet pieces. The champ style can be used with the right kind of car - i wouldn't do a model 'a', proportions'll be all wrong. Another problem with the champ style from ebay is they likely have a spool and you'll need a locker to drive it on the street, so you'll have to spend more money.
Sprint car QC have live spined axles, in brief, the axles isn't enclosed and it has splines on the end and the floater hub is attached to the spline, so it has a special type of wheel. Early sprint car QC's are champ stlye and have a disengagement means so the driver (direct drive) can get the car to freewheel or coast.
For the traditional look you really need the Ford V8 style so's you can bolt the bells from a banjo (not model 'a' banjo, won't fit - diameter/bolt circle is different).