I might be the only guy on here that says build the engine first,but that is usually my way of doing it.It has more to do with the fact that I build race cars as a hobby and horde engine stuff,so I tend to have piles of good engine stuff laying around waiing to be built,but it always seems to work out well for me.The fact is that in my case I usually dont have anywhere to keep another car that I cant use to it's potential.In your case you are dreaming about a 10 second early nova.If you buy the car first your reality will be that you will own a dead slow straight 6 or small V8 turd that you have no interest in driving and no place to store.Best case you are looking at 100 bucks a month or more to store a multi thousand dollar investment that you have no use for.I say build the engine,build the trans and build the rear.Buy all the fuel system parts,the wheels and tires,the seats,and anything else you can think of.The truth is that you can hide all these parts in an apartment if you got hard up for space,and have a few buddies to help you lug it up the stairs.Buy the car first and you will be out of a bunch of money,still not have anything you want,and you will have an awfully big pile of something that will require an awafully big daily commitment to own.
Another factor in this equation is the level of car you are looking to build.You arent talking about a basically stock car with a warmed over crate motor and a stock posi rear in it.You are talking about a 600 hp 383.In a 3200 pound car that kind of power{ at least if it is normally asperated} is best matched to a set of 4.88:1 rear gears,and a 4500 stall convertor.You cant exactly stab a spooled 4.88:1 rear behind a 6 banger or 283 and figure that it will still cruise like it is stock.The car will be slow AND undrivable.Same thing goes for that full manual valve body th 400 or 'glide and the 4500 stall convertor.Dont waste your time on a $300 set of 2" duals on that stock engine to spiffy it up because the big dawg you are planning for is going to want some 3.5" pipes with turndowns and some 2" primary tube headers.
Think of it like this.You need to make a huge investment to own the car that you want at the level that you want it at.No matter how you slice it you wont truly be happy until you have spent every last dollar that you need to spend to make the car exactly what you want,and however long it takes for you to gather that kind of money is how long you should plan on waiting before you get behind the wheel of your car.If you do it my way you could have most if not all of the mechanical parts gathered and ready to go before you ever go car shopping.When you find and purchase the body that you want you will still have to have it sent out and have a roll cage and frame connectors put into it,and any other body/paint work you want.Even if from the day you buy the body until the day you get in it and drive it away takes a full year,you could in no way have ever expected to get it done in any less time had you not gathered all the mechanical parts first.
And one last thing.Buying the car last leaves the most exciting and important part of the project until the safest time to execute it.It sounds like you are a chevy guy,or you at least like GM stuff.If after a few years of piling up parts you decide the early Novas dont float your boat,it wouldnt be a stretch to build any of a dozen or so other types of GM cars that most of your parts would fit into.If you get the car first you will want to enjoy it right away.You will go out and buy a bunch of stuff to make it more enjoyable,and none of that stuff will be anything that you really wanted on your finished project.Doing this will set you back years in the gathering of the parts you want,which means that no matter how you slice it,you added time betwen you and your dream.Take it from me,hindsight is always 20/20.I learned a lot from all the junk I built in high school,but had I known then what I know now,I could have finished senior year while driving a totally badd azzed bigblock powered 73 camaro with a fresh red paint job and 4.11 gears,but intead for the same money I ended up finishing out that year with a non running car with a primer body because I made a handfull of attempts to keep the car running with junkyard parts and "just for now" fixes.That was more then a few years ago,but it is just one pretty good example of how easy it is to misuse time and money on a project car and end up totally sidetracked.If nothing else,during the entire time that you spend gathering parts,you happen to luck into the deal of a lifetime on a car that you want,you can buy it then.If you buy the car now,I would be willing to bet that by the time you are finished with it you will have found a half a dozen other better deals that you had wished you waited for.
Good luck.