A lot of owners on here preach "find the camper, then get the truck to haul it". This thread seems like you want ideas to convince the owner to sell it to you. You could ask him to deliver it to your home because you do not want this gem to get away while you shop for a new truck.
Also, I looked up the 97 brochier to see if there were any specifications for cab length put out by lance and found a little info. Obviously they talk about weight, but they also show pictures of the said camper on a crew cab, extended cab, AND regular cap trucks. You could mention to the seller that since you only have a single cab, this allows you to have more payload. Here is the link...
http://www.lancecamper.com/docs/Lance_1997_brochure.pdf