Caution, long-winded opinion by individual lacking any objective unique expertise in this matter follows:
That type of question depends upon your point of view. At one time, a 67 Camaro was a collector's item, but a 71 wasn't. Some people now are claiming that late 70's personal luxury coupes of the malaise era are becoming classics. Eventually, any well preserved original vehicle will generally be worth more than a modified one. A classic requires two things: scarcity and desirability. It is common for sellers to overlook one or the other when pricing their vehicle for sale. Often somebody will buy anyway. Except in the case of customizers who sideline drawing demented rats holding onto out of scale shifters, originality always ages better than customization. In the early 90s, everyone wanted a shaved pepto bismol pink or lime green pro street style vehicle, and the ideal mini-truck had a neon palm tree in the back window, and an off kilter sunken license plate. The 90s were not unique in this context: two words- Corvette Summer. Once upon a time, a person couldn't give away a station wagon, now they are a desirable body style.
Although hard to believe today, a 305 chevy truck will someday be rare. If it is also desirable, it will be monetarily valuable. If someone wants to be the curator of that particular piece of history for the future, then by all means, keep it original. If they want a 350, then that does not make them a bad person. It might make sense to see what it costs to get a rebuild on the 305, compare it to a 350. If you have a kid, choose the cheaper one and put it in their college fund. If you tow, a 350 is better. If you TOW, a 454 or diesel is better, but then expect to upgrade parts downstream. In terms of ensuring that your truck is a rare original, the best odds of ensuring that is rare is to keep the stock ride height.
If a truck these days is not 2 inches lower, it is 3 inches higher.