Second choice would be a mild V8 transplant, and here is where I will have to soundly disagree with some of the advice given. A full conversion to 5 lug etc. is NOT mandatory.
I'm on the same page, but I think some clarification is needed for owners of 1965/66 I6 cars who might be reading this thread.
All base line 1967 and later cars have the same front spindles and an 8" rear end. Obviously, the hi-po cars got upgrades, but that's irrelevant to this topic. So besides new springs front and rear, there's not a lot to "convert" when swapping a 1967 or later car from an I6 to a mild V8.
The 1966 and earlier I6 cars are another matter...
Since the 1980s, I've heard many horror stories about the weak 64 through 66 I6 spindles failing while driving and eventually being recalled by Ford. To this day, I'm not a fan of daily drivers using those original I6 spindles. Don't even get me started on people still driving around with the original single-reservoir master cylinders!
I also know from personal (scary) experiences how badly those tiny 9" 4-lug drums stink. Once you hit 50+ mph, you better think way ahead when it comes time to stop.
Finally, the 7.25" rear end the 65/66 I6 manual cars got is just truly awful. Dumping a true 200 lb/ft of torque into that weak rear end will likely just spin the rear tire, but the rear end will break eventually. How and when it breaks can get ugly. Maybe you'll get lucky and just snap an axle, but locking up a rear diff at highway speeds is bad.