[Greasey Harley]

Properly installing a small block in a formerly l6 truck will never destroy it.
As long as It's done properly it won't hurt the value, and [in all likely hood] will probably increase the value.
6 cylinder base model pickups are not particularly rare. It's intrinsic value is that it's an original short wide.
Do it right, do it nice, make it the way you want it. It's your truck. Doesn't sound like you're planning on selling it anyway.
Nice truck!!