[Bryn Tirion]

Remember that the OP was about an effect on value of the car with an original engine as opposed to the 340. There is indeed a positive value in having a Bristol-sanctioned drivetrain, which can be: a. a 313 poly; b. a 318 poly;c. a 383. The transmission could be either earlier or later push-button Torqueflite, or 727 Torqueflite. For 1965-level performance there is little to beat a poly engine. The 408 313 develops stock 250 bhp at 4400 RPM, and that is straight, unmodified 1958 Plymouth Belvedere with AFB. Bristol pretended they tuned it, but specs are Plymouth stock. The poly A engines developed about 15% more bhp with comparable carburation than did the later LA engines. They're just, as Thor says, heavy.

Since it is rkirk's car, he can do whatever he wants with it, but to fit in with the smooth, high speed cruising style of the car-as-built-in-1965, the 340 or other high cam engine is unsuitable, and a 360 is not a Bristol-sanctioned engine. At slow speed, these cars are meant to glide along almost silently, with just a subdued blub-blub from the tailpipes.

The early cast iron Torqueflites are heavy and run hot, and the aluminium 727 is a much better unit for those reasons. If I could keep the pushbutton operation, I would make the change, but I am not aware that you can.

As for making the 408 into a more modern drive, there lies the slippery slope we can easily get sucked down, and I've been that road myself. Your 408, almost no matter what you do to it, will not beat my 2004 Focus wagon with 2.3 litre Z-Tec engine to 60 mph (7.8 seconds), and that's not fast these days. So why bother? Keep it as a 1965 car and enjoy the ambiance. It still drives very well indeed, feels smooth and powerful, and will easily keep up with modern traffic under normal driving and at any legal speed, plus 40 mph or so!