Tell us what you're working with. (car, transmission, rear gearing, engine, carb etc). Gearing, transmission can be big returns on change. Some things will make a "street" car more "fun" to drive, and other changes will make a "strip" car be quicker. So, it helps to know what you have and where you want to go.
Best bang for the buck would be nitrous! x2 on the transmission and gearing changes. But get the carb, distributor timing, exhaust, and add a low restriction air filter first.
Iv'e always wanted to post this and also show the importance of a good exhaust system on a Nailhead and just how much any exhaust improvements help. When I bought my 66GS in 1979 while in high school I didn't have much money. I was running just the 2 mufflers and no tail pipes for a while. The Police didn't bother me as I guess I was a good kid?? The car came with really sticky recaps tires. It would spin these tires for maby 10 to 15 feet with the ST300 a posi and 3:23 gears but these re caps stuck like slicks. I finally was able to get some tail pipes and then it wouldn't spin these tires more than 2 feet after. Yes it was hot out and in the winter it gave some better burnouts but just showed me how much help they need on the exhaust side.
First off and more importantly what condition is the motor in? High mileage, over 70K? Rebuilt top to bottom less then 4K miles ago? Any signs of knocking or pinging taking place? What's it average blasting down the highway at 60 plus mph coolant temp ? How much mechanical aptitude do you have?
63 buick electra 225, 401 nailhead car is all stock as it came from factory. Not looking to race just a nice street car with some power
If you want want greater and long lasting power then it's time to bite the bullet and rebuild it, as opposed to asking for more power out of something that's well in the process of wearing out. Those 1963 plain cast iron compression rings loose a bunch of needed tension even with just 50K miles of normal usage on them! To me it's like asking for meatball that's been frozen for 3 years and with freezer burn all over it to taste good once you defrost it! Also consider the plane fact that these days the longer you wait to rebuild a nail head the harder its going to be to find parts for and for certain the cost will skyrocket with passing time! There is two otter things you can do,1) get the car dyno tuned and after that if you still want more then give serious thought to installing stepper rear gears.
Aside from a fresh motor (a big plus) the trans change was huge. On my ‘60 364 LeSabre I switched to a 4bbl with doc’s mod, dual exhaust, and distributor. Biggest performance difference was fitting it with a switch pitch 400 using st300 converter. Felt like I put a bottle on it!
at 140K on my 62 LeSabre, I went from a 2 barrel to a quadrajet. Pretty noticeable difference. But I have to echo what others are saying about a rebuild. I took my engine out last Fall for a rebuild. The timing gears were a ticking time bomb. The nylon gears were completely stripped. Also, the distributor weight bushings are gone meaning there was massive hysteresis in that system and really no hope of getting good ignition timing. I also found that whoever replaced the head gasket used the thicker Felpro gasket versus the standard thickness gasket. So I probably only had 8:1 compression. I understand the temptation to save a few bucks. But sometimes going the extra mile is really the cheaper way to go. I have no doubt that some time very soon, I would have been rebuilding this thing anyway and having to cope with catastrophic failure.
