Howdy Slideordie:
Sorry to be so late in response, but we've been on the road. After catching up I usually do some back tracking. That's when I found your post. Be sure to start any upgrades with a top notch ignition system, a good carb and clean filters.
For a stock '65 200 Comet, the 1st and best, free upgrade is to add another 5 degrees of initial advance to the distributor.
2nd would be to add a performance 2" exhaust system. The time to do this is when you need to repair/replace your current system. 2" pipes and a matching turbo muffler will yeild better throttle repsonse, performance and economy. About $200, but you need a safe exhaust system.
3rd would be to swap the stock 1100 Autolite for a 1101 from a '64 Ford with a 223 engine. It will be compatible with your stock Load-a-Matic distributor as it will have a Spark Control Valve to complement your Load-a-Matic distrbutor. It may need adaptation to the extent of slotting the holes to fit the narrower mounting studs and tapering the adapter to accomodate the larger butterfly valve. Make sure the linkage affords you full throttle when the pedal is floored.
These three mods will make a significant difference in performance, and properly tuned, will not effect mileage- may even improve it. Now is the time to consider the $$$ for a Petronix Ignitor, which may not improve performance, but will improve starting, idle quality and eliminate points and points tune-ups.
Please keep us posted on your progress and successes.
Adios, David.