Ah, if most people took that path it wouldn't be nearly as fun.
Practically speaking, however, if you've got the DNA for it -- shoulder belts are easily available retrofits, the MPG on the old straight sixes ain't bad, and the maintenance on a car that has nothing but an engine, and alternator, and a battery under the hood is pretty low.
As a matter of fact, I'd bet my total cost of ownership, including fuel, purchase price, dealer visits, and depreciation would kick butt compared to most newer cars.
I'm driving a straight six '64 Chevelle right now. Picked it up 2 1/2 years ago with 23,000 miles for $4500. Estate car that was stored in a clean garage for nearly 40 years.
The interior looked like it had never been sat in, and it drives like a new 1964 car.
Cost me replacements of 4 dry rotted tires, a fuel pump with a split diaphram, new brake lines, plugs, and wires, and points in the first month.
That's it.
(Well, I did swap out the 3-on-the-tree transmission for an E-bay one with the Borg-Warner electric overdrive tailstock. Brought the highway MPG into the 20s at 70mph.)