John,
The Cadillac 365 engine had oiling issues, particularly at the rockers/valve train. While today's oils might alleviate this problem, it remains an inherent flaw in these engines. Most guys upgrade to the 390 engine, which looks the same, but does not have the oiling problem.
A modern engine/transmission combination means that you will be able to drive the car, anywhere, without the fear of needing a part that would not be stocked at any AutoZone or Napa store. Try getting a fuel pump for a 331, 365 or 390 Cadillac off the shelf when the car breaks down... Unless you happen to break down in front of Kanter auto, or the like, you will be at the mercy of UPS or the USPS to ship you one.
Yes, this car belongs to someone else now, and yes, it is his to do with as he wishes. Even though he was provided with a freshly rebuilt 390 Cadillac engine, he chose to go late model.... his choice, his right.
I do not hesitate to drive my old cars anywhere, at anytime, but I do take an obscene amount of "just in case" parts with me, and even then, something, anything can go wrong.