I have one of these on my GTO.
My Holley book says this about that carb:
"Competition replacement for all Quadra Jet carbs on 1965-70 327, 350 402 cid engines. Mechanical secondaries, spread bore, double pumper 650 cfm, non emission."
So this is a well-suited carb for your engine. Personally, I like the vacuum secondary design more for steet use, but most spread bore Holly carbs have the mechanical secondaries.
Holleys are easy to rebuild. Just follow the instructions that come with the rebuild kit.
I would suggest replacing the 'power valve' The rubber diaphrams in these can get old or blow out due to a carb backfire and start leaking, which makes your carb run really rich.
Also, All Holley carbs run too rich from the factory, unless you have a really wild cam in it.
Someone has installed a secondary metering block on your carb. This is a really good thing because it makes jetting the secondary carb a snap.
When you disassemble the carb, look at the numbers stamped on the jets. The primary side should be a #62 and the secondary #65.
(Dont mix up the jets). The higher the number, the richer the mixture.
If you have a mild cam, consider getting new jets about 1 or 2 numbers lower (leaner), and your car shoudl run better with better mileage.