I agree that you should buy a muscle car for fun and not worry about its future value in 20+ years. When I buy my Challenger SRT it is going to be driven!
One point about the horsepower of the 1970-1971 340. Plymouth/Dodge rated it as 275 gross hp at 5,000 rpm to get a favorable bracket at the track. This is because the factory stopped taking hp readings before the engine redlined at 6,000 rpm. NHRA figured out this ploy and factored the horsepower up to 325 hp. If you use a net rating for the 1971 340, it would probably be around 275 hp.
The net 30 hp difference between the 1971 and 1972-1973 340s is due to the higher compression (10.5:1 vs. 8.5:1, bigger intake valves (2.02") and larger X heads (the 1972-1973 had smaller-valved J heads).