Craig,
I have been checking your rules and it seems like the path of least resistance for many. The 76 Vette is the cheapest but all the mid to late 70's cars or plentiful. GM manufactured over 542,000 gen 3 corvettes. So any 68 to very soon the 1976 models meet your exemption. The collector or bumper cars were made until 72 with the 73 and 74 even if big block equiped command $$ if not pure stock.
Another part of the magic path is a US supplied Vette comes with a lower US value or purchase price. I understand the vettes in country already command a much higher price than the total after import. If 30 yrs old the duties, taxes and other fees on a US vette are reasonably low as a total cost.
The 98 inch wheelbase and 60 or 61 inch track width with all the stock goodies make it a hell of a ride and still about 2400 pounds. They even came with side exhaust and aftermarket chrome set looks even better.
PM me at
jeffc@cobrasnvettes.com for further info if you would like to hear what I found out. I would love to provide you a kit and MSO but will be happy to help you build your own version were I can.