What part(s) have you let go, only to wish you had kept them not long after?
If we're going big... the 71 B'cuda ragtop I sold in 89. Originally a 318/auto with AC, PS, PB and a manual top. Cloned to 340/auto with a rallye dash and maintained the AC, PS, PB.
Or the bare bones 383/4spd 69 Road Runner that I bought the day after an put a 440 in.