k, if you're running headers, all of the pipes from each port come together in a collector. they come to a 3" 3-bolt flange at the end. so, in order to add exhaust tubing, you need to put on a reducer. they come in various sizes, but most generally will bolt onto your collector with a 3 bolt flange and then allow you to mount any pipe you want off of your header.
after that piece is on, you run your exhaust pipe (flex, or custom-bent) from that reducer to the inlet on the side pipe. this is where the exhaust enters the side pipe...if you're using corvette side pipes (i'm assuming from a 60s car) it should already be 2.25 or 2.5 " diameter for the inlet size. in this case, you should run a 3" to 2.25" OD header reducer, then 2.25" ID flex pipe (ID so it will fit over header reducer and inlet on side pipe) to the side pipe...this will result in the flex pipe simply sliding over your header reducer and flex pipe inlet...clamp it on and you're basically done.
*Note: if it's actually 2.5", you would use a 3" to 2.5" OD reducer and 2.5" ID flex pipe
It's really pretty simple...just a lot of "persuasion" if you know what i mean
