Alcapulcoman

I'm considering selling a 1969 Mach 1; 428 R code/C6. It's a very decent looking car but it does not have its original engine and trans and the paint was last done 35 years ago and close to a re-paint. Any suggestions on how you factor non-originality and possible paint needs into a sales price, %'s or amounts or none of the above ? Thanks.