Hard to say without seeing the condition of the car, but, it looses a LOT of value without the Boss motor....unless you happen to have a Boss 351 motor laying around in your garage...
It would not be worth much more than a Mach 1......
Figure a good chunk of change say, 8K for a replacement, period correct engine if it can be found (just an estimate!) Then you get into the Boss351 specific items which are really hard to find. Also, check real close on the condition of the rear shock upper mounting area, these were often ripped out. They're a beautiful car and if you have a lead on one I'd love to hear about it! I can take 10 years to accumulate all the parts /forums/images/icons/smile.gif
Bottom line, there's not a lot of premium on the VIN alone because the cost items are all mechanical.
Just my opinion,
Its value really isn't any different than a non-BOSS car with a Cleveland. Yeah, the presence of the R-code VIN tag has some value, but really only to someone who intends to do a full on restoration. This would be especially true if there are none of the original, unique parts that go along with the car.
You mentioned the rear shocks are routinely ripped out on Boss 351's, well mine is one of your examples. I need a new trunk floor anyway, is that the easiest solution, e.g. put a new trunk in and bolt up to it?