The deeper pan which drops down is probably a mistake for the street and may always be a concern and need a watchful eye as to road conditions. It may have been a better choice to go with a pan that has the side kick-out that adds the extra oil capacity. It still has a low profile and won't hang to low. https://www.jegs.com/i/Canton-Racing...iABEgLMc_D_BwE
You may want to decide what you are expecting out of your suspension and its ride quality and then build the suspension to meet your needs. There are many ways to get to where you want and this has been covered on the forums several times. Different members have gone different routes from pro-touring coil overs to stock rebuilds. The $$budget may be a deciding factor as some options are not inexpensive.
I suggest doing your research and read a few online articles and perhaps get a book on GM suspensions and handling. Arm yourself with the correct knowledge so you only have to buy your parts once and get the ride you are looking for. The GTO suspension is the same as any GM A-body and you may find a few more articles under a Chevelle search.
You have many options: sway bars - stock or oversized; bushings - rubber or polyurethane; front A-arms - stock or tubular; rear control arms (adjustable or non-adjustable) - rubber bushings, polyurethane, Delsphere, & Rotojoint; springs - stock, HD, custom, lowering or raising; shocks - stock oil style, gas, adjustable like the QA1/UMI/Blistein brands.
Everything needs to be matched and paired together so it all works in unison and gives you the handling & ride you are looking for. Many of the big name brands offer a complete package where everything is matched to work together and takes the guessing out. But, you want to know what it is that you are buying - so knowledge of the different offerings and material types can provide you with a better decision when the time comes to order your parts. What one person likes and recommends may suck for another. So get informed and then go from there.