Here's what happens when your Tesla Roadster (or any EV) reaches the end of battery life

Here is a comparison I like even better:

Used 5.7 motor $2,000
8HP70 $1,000
Swap parts $1,800
PCM/Harness $2000
Suspension $3,400
AC $3,000
Wheels/tires $1,000

That leaves $5,800 to find a reasonable shell to put it all in and stay under $20K.

That would result in a fun car to drive and would return 25 mpg average, or better.

74 Duster/5.7/8HP70 - $20,000
12 years of fuel - $28,800 (144,000 miles @ 25 mpg @ $5 gallon)
Total - $48,800

Note that I am using $5 a gallon for fuel costs as that seems more realistic for my area in the future. Current prices are around $4.60. The $5 fuel cost compared to the $3.75 price above is a difference of $7,200.

And if I am ok with a more stock suspension and didn't care about AC, I bet I could knock the vehicle "purchase price" down to $14K or so.