What I am able to tell you from the mid west dirt scene.....using the rules that I still have from the late 70's thru out the 80's.....
The Hobby Stocks, by the late 70's, where so close to the Late Models that at some tracks they where called Limited Late Models. Here locally (Topeka, Ks. / Kansas City) the Hobby Stock class had been dropped in 1980 and renamed Modified Street Stock which ,by the rules, made for more of a division between the Late Models and the Hobby Stocks. By 1986 they where being called Sportsman....Camaro stock front stub with the rest of the frame and cage more or less matching their Late Model counterparts of the day.
As far as your questions.....
Engine: In 1979 the Hobby Stock rule was 7 1/2 Lbs.(car weight) per cubic inch along with a $1500.00 engine claim ...it was put in place to discourage spending enormous amounts of money on an engine.....keep in mind there was still some of the mind set that there was no substitute for cubic inches and cubic $$$$ .
A 400 small block Chevy would have been an engine of choice ,so what ever you want to use engine wise would be plausible...going low buck I would lean toward a small block since they where a dime a dozen back then.
Since the Nova frame is almost identical to the Camaro, put in a pair of sub frame connectors and "X" the frame as you would do any stock car. Use the Leaf springs as you would not want to mess around with a coil spring set up and the cost it would entail switching over from leafs to coil springs or visa versa if your dealing with a Chevelle. This class of car would not have had a quick change.....use the kit rear end or a Ford 9" ...a very early 80's car could have also had a Dana 60 rear end.....cheap, durable and plentiful.....by the mid 80's the Ford 9" was pretty much the rear end of choice.
Tires: Another rule that varied from track to track.....early 80's 8" tread...that's the tire print on the surface, not total width of tire from side wall to side wall....by 1984 rules simply stated 10" max. WHEEL width...so what ever you could get to fit a 10" width wheel was good to go.
Some other rules,1980 on.....Headers where o.k, by the mid 80's they had to have collectors, no Zoomies. Cast Iron intake manifold as David stated. Completely stock frames, stock springs and stock upper and lower control arms, front and rear end. 4 shocks in stock position, no weight jacks. By the mid 80's racing shocks where o.k 6" rear deck spoiler, full factory floor pan and fire wall. you could use Black plastic along the rocker panels and below the front bumper.
Any other questions, don't hesitate to ask.
