So figure out how much a V8 conversion would actually cost to do, and maybe offer the drivetrain upgrade option for $$$$ above the sale price for the given motor/trans conversion, and see how it flies.

Like the others, I think the slant 6 is just an excuse for them not buying it, but if the conversion offer was on the table for less then they could get it done themselves, it might be enough to tip the scales for a dreamer

For the average guy off the streets, any inline 6 had a reputation for making the car a no power slug, weather it was true or not. Those people would have no idea where they might find someone capable of converting to a V8, nor do they have any idea how much it would cost. You giving them a conversion offer with a specific motor & trans (not "a" 318, but "this" 318), a detailed conversion plan covering what was going to be done and what was not going to be done, a price tag, and a time frame on when it would be done, might make the car much more attractive to some. I wouldn't undertake a swap until you knew some one had the money and was buying the car, and knowing you can accomplish the job in the given time frame at the given price. I would also expect your labor rate to be well below the going rate unless you are an actual cad caring mechanic.

If I were contemplating going in this direction, I would expect to get very near my asking price for the car, and I would expect to cover all the expenses of the conversion. The time doing the conversion would not be compensated for. How bad do you want to sell the car, and how much is your time worth to you? You may be farther ahead to reduce your price and sell the car as is, or wait until the right person shows up to buy it at the current price. The conversion is a lot of work, especially if you have never done one.