Think of the hardtop as a "mod" on the car (since it was optional equipment). Selling with usually only generates about $0.20-0.30 on the dollar of new... even less in this case I'm afraid. When selling separately, i.e parting out, you should be able to recoup 50-70% of the cost when new.
So, including with the sale might help "sell" the car but nominal increase in selling price. My recollection says hardtops go for $1300 depending on condition, color, etc. You'll never see that much increase if bundled w/ car vs w/o. You can try and keep it as a separate cash transaction which will also help buyer for sales tax purposes. However, that will be very difficult once you disclose it b/c as a buyer, I'll be inclined to immediately put that in play during negotiation. To get the most value you might want to try and sell hardtop now or hold until early fall when demand is up. Summer certainly won't be the time unless you'll be advertising it focused around track junkies.
Good luck and what p-car are you replacing with?
