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Looks like a typical 390 block. The 66-427 rear bulkhead pattern is meaningless, it shows up on 352's and 360's too. I would have it magged before proceeding.
I would go beyond a mag. Have it sonic checked and the crank saddles checked and align honed as well. I just had a 176 mile engine in my Shelby start putting coolant in the oil. I had to pull it and sleeve #6. Sadly, core shift, corrosion or incompetent machinist can cause a myriad of issues. It's no minor task to pull an FE with headers, especially post a 15K paint job. Do NOT short cut anything. -Keith
Yes good advice. Even 390's sometimes have very thin cylinder walls. You never know unless you know.