First, welcome to CHR! Glad you're here, and looking forward to pictures of the Chevelle.

On your oil leak question, it's all but impossible to diagnose blind. Like Jack says, it's very, very easy to over tighten an oil pan and squeeze the gasket out from between pan & block. Also, the joint between the pan rails and the front & rear at the mains can be a nasty spot. I like the FelPro one piece blue rubber pan gaskets that come with the four plastic retainers to hold the pan & gasket in place while you take your time to put the other bolts in place. Cleanliness is critical with them - no residue, and clean all surfaces with solvent, like carb cleaner or brake cleaner spray.
You changed the intake, so another easy oil leak is the front or rear valley seal. I throw away the rubber or cork gaskets that come with the intake gasket set for the serpentines front & back, and after sticking the intake gasket to the heads using something like Gasgacinch I lay down a heavy bead of silicone, at least 1/4" OD and let it tack for about five minutes. Once tacky, set the intake down straight, zero lateral movement, and torque it into place. Alignment bolts at the corners of the intake help a bunch with dropping it down straight, with no jockeying for alignment.
The other one that's bit all of us at one time or another is simply missing a hole on a rocker cover gasket, or like the oil pan, getting too strong on the ratchet and squeezing the gasket out. It's amazing how much oil can leak out of a rocker cover when the gasket's not aligned.
Bottom line, if the car didn't leak oil before then it's all but certain that something you've done introduced the leak. I can't see how you would have suddenly developed a rear main seal leak with what you did. Backtrack and look at all of the places you had stuff off and/or loose.