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You've had this car from almost new (bravo to you) and you plan on changing all the valve train so why are you worried about the "value" of the heads? You've had it this long...A little bit of milling does the SD-455 a lot of good - especially when combined with a set of JE aluminum pistons ( don't have the part # handy but I will look it up - SRP also makes a good SD piston #149721), and nobody but you has to be the wiser. Here is an old school article on an SD-455 '73 T/A and the work that went into getting it to go faster using stock parts. It is old tech but still some valuable information.

A gentleman by the name of Steve Shappaugh has forgotten more than I know about getting Super Dutys to perform and he would be worth talking to - look him up online. It's a great engine and with just a point of compression, a good ignition and traction aids, it is a "real street sweeper".