My two choices.
TOP: 1970 Mustang Mach-1 428 DOHC A monster in its day.
2nd: Triumph TR3 convertible. Because I don’t mind taking out a wrench and working on a car on the roadside every few miles.
Triumphs were all fun and games when they ran, which - as you say - was best described as "occasionally."
I remember my dad standing beside a (non-running) Triumph Spitfire and saying, "It makes you wonder how any British airplane ever made it across the Channel during WWII."
I had a Triumph TR2 convertible. Wish I still did.
Probably nothing a VW 1.8 turbo wouldn't fix, or even a lowly 2L SOHC.
Another popular transplant is the GM 3.2L 60-degree V6. It's a narrow block and can be found in 1990s model Camaros....complete with a T5 stick.