Yep, 1969 vehicles are just like fuel injected ones with computers.
Bigger injectors aren't going to "help the performance a lot" unless you need them and are experiencing higher IDCs than you should be, causing the injectors to heat up and become damaged and/or fuel cutting out. They aren't a power adder, you get bigger ones if your injector duty cycles are above 80% quite a bit at wide open throttle.
Injectors aren't meant to change the way the vehicle runs at all. You
need to adjust the injector constant (decimal number which tells the PCM how large the injectors are so it knows how long to hold them open for and adjust the air-fuel ratio in closed loop properly). If you put in any size larger injectors without changing that number, your closed loop performance will be off, and it will cause the car to run badly at idle and low throttle conditions. Open loop (heavy throttle) will be too rich without the injector constant changed as well.
I am extremely surprised that, especially for an N/A application, where you want to extract the maximum amount of power from the engine, that you do not have any tuning software. The tune is vital, it's
everything.