The front section of the driveshaft will be shorter on the 1800 due to the OD box, which is good. Unfortunately, the rear section (from the suport bearing on back) will also be shorter - that's where the 1800 is 'missing' wheelbase as compared to a 120. You might be able to mate the short front section (which should be the correct length for an 1800 or a 120 or a 140 or a PV) to the longer rear section you already have (which would work for a PV, 120, or 140 - they all have the same wheelbase) if the splines all match. Theoretically (OK, not so theoretically but actually) the front and rear sections are balanced as a unit and should be rebalanced if not remaining in a set.

Toss in several sizes of flanges on the trans and diff for more fun. I somehow got exceedingly lucky and got an M41 with the proper size flange for my PV - I got the PV shaft cut down by 11 1/4 inches at a driveline shop for a very reasonable price ($35). They wouldn't rebalance mine as the splines had a tiny bit of slop so I used it as is and haven't notced any vibration.
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I'm JohnMc, and I approved this message.