The driveline shop left the upper mount in place, and I haven't bothered with it since. At the time of the conversion I wasn't sure how the longer lower hanging d-line would work on the trail, so I wanted to have the option of going back to a 2pc if I found the need.

IIRC there isn't that much room between the skid plate and the tank, and even a small indentation can reduce the capacity of the tank. This may not be a problem for you, but if as long as you are willing to replace the tank with a used one, the machine shop shouldn't be unwilling to pay for one, especially if you are willing to cut them a break by doing teh work yourself. Along with the letter, I would send them copies of 3 estimates of what it would cost to have a shop do all of the work, including a replacement d-line. This may also help to sway them to pay up for the what you are asking, after all you don't have to do the work yourself.

If they haven't returned the check to you yet, make sure to put a stop payment on it, you never know they may deposit it after recieving the letter just for spite. Also make sure to send the letter registered, if you have to take them to small claims court you want to have documentation of everything from the initial work order through trying to resolve the matter w/o the court.

For a little more info on my 1pc d-line, I re-measured it today, and it is 67" long w/a 3.5" diameter, so the machine shop is wrong when they told you a 60" d-line is the max you could go.