IMO, the 67 and a newer lx car are not interchangeable.
I bought a mustang gt in 2012, but it's not a substitute or replacement for my 68 satellite.

For DD duties, I just consider those vehicles "appliances".
All I need from it is darn near 100% reliability, good mileage, and to not "dislike" driving it.
My current appliance of choice is a Honda civic....... I've had it just about 6 years, been a great little car thus far.

If your only choices are keep the 67 and buy an appliance for 6k, or sell the 67 to finance a srt8 300........ I'd opt for the appliance and keep the 67.
But a big factor in your decision would be how attached you are to the 67 and how well you like it.
The newer stuff just "seem" more like vehicles you don't get real attached to.
Use em for a while and move on.
The old school stuff kind of works itself into your lifestyle

I really like my mustang......... But if it went away, I wouldn't mind that much.