I'm no expert on this as I've had my car for nearly 20 years. Have spent a fortune on it and drive it like I stole it with a grin on my face. That pic < is my wife using it on a track in the middle of a rally across France.
I feel blessed that "value" has increased along the way as it makes the man-maths easier. But in a global recession, I think those days are numbered.
At the price point you mention, and now knowing the block is not the original 230 block, the question I'd be asking is "is the juice worth the squeeze?" - what price your "upgrades" (eg disk brake conversion - given you have the hubs today) and any knock onto "value" - the latter being in inverted commas as if it's your driver/keeper, then value is in smiles per mile. If you're a dealer/flipper, value is what someone will pay. If you have a 280 engine, use that.
Essentially, and being brutal for sake of clarity only, if it's rusty and running like a three legged dog, disk brakes and a shiny valve cover will make no difference to what I read as you meaning by value. If YOU prefer it to look stock and that's your value, make it look that way. or compromise on your preferences if the juice is no longer worth the squeeze... From a market perspective, today a #3 car with a non-original block is not suddenly going to become a #2 car with commensurate value by having "280 upgrades" unless it's running perfectly, is solid, reliable, etc etc
Have a look at the signature of Benz Dr (Dan Caron) on here. His red-rocket is a bit of a mongrel of a car on paper and not "stock". But if he came to sell it...
When is a mutt a cockerpoo?
One thing I've learned on here over the years, is that someone will be able to make the opposite argument very eloquently...