What All High Milage Issues?

I recently got my stang back on the road after parking it for like 5 years. (2001 GT) After it somehow passing Cali smog with stage 2 heads, cams, headers, ported upper intake elbow and sitting on a spacer (they didn't make upper intakes when I built this thing) with 75mm TB and a C&L MAF on a JBA dyno tune for similar setup (minus heads with a stage 2 cam) I have been running into some of the issues that I keep finding posts on like the first time I filled it up I blew out the fuel filler gasket! X(

Upon first getting her going on new fluids: I had codes for my oxygen sensors that I cleared and never came back (think mixture of good/bad gas from sitting), a code for too rich (to get her going we had to turn the key to on like 6+ times before actually trying to get her to try to start with like 15+ minutes until she finally did) The long block is a custom build with only about 1000 miles on it and it has a "Performance Automatic/PA" tranny with a 3200 stall convertor and only like 10,000 miles on it.90k only on the car but it has been sitting in the worst type of place for storage.

If you have ideas on specific things to check then it would be appreciated since I am about to sell my Corvette within the next couple days leaving me with no other daily driver.
 
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