You can do a Google search and find a better description but basically you have to pair certain cylinders together.
This picture shows a set of Tri Y headers with a regular Chevy firing order.
These days it's getting popular to use a cam that has the #4 and #7 cylinders switched in the firing order. IF you happen to have one of those cams, you'd have to pair different cylinders to get the best effect out of the Tri Y headers. It's not likely your engine has a 4-7 swap cam unless it's been recently rebuilt because these cams have only really become popular in the last five years or so.
You can check by following the wires from your distributor to see if it follows the standard Chevy firing order... 18436572
I ran your specs through the Pipemax program and here's what it says...
First Y segment... outside dia. 1.572" length 18.1" to 19.7" long
Second Y segment... outside dia. 1.697" length 18.1" to 19.7" long
Collector outside diameter... 2.443" to 2.693"
Best collector length... 22.7"
The collector length won't matter if you're just reducing it down to a smaller exhaust size but if you're going to put in a cut out or a crossover pipe it would be best at 22.7" from where the secondary tubes end.
Your engine is pretty mild so you shouldn't expect a huge bump in power with the headers, but they will help a little,,, and look cool.