I started off with a 122 tunnel, and found that it was a couple of inches too shallow - apparently the floorboards are higher in relation to the drivetrain on a 122 than on a 544. I had to cut off the lip on the tunnel (that mates with the floorboards) and make some fairly wide angle iron shaped sheet metal strips to extend downward from the tunnel to the 544 floorboard height. All in all it took 3 or 4 angle grinder discs and a spool of MIG wire to get mine together.

To keep thngs simpler I didn't mess with the existing trans crossmember mounting locations or the junction of the tunnel to the firewall. I just cut downward at the midpoint of the shifter hole on both pieces of sheetmetal, then bent the 122 tunnel inward at the front until it mated with the narrower 544 tunnel.

An 1800 might very well have a lower floorboard height (not a huge concern really) but another consideration is the rearward 'short' shifter placement. While it would certainly be a nice featuer, by my estimation it would put the back end of the tunnel into conflict with the hand brake lever. Not an unsolveable problem though, possibly not one at all. Of course, not an issue with a 444 and its dash mounted hand brake.

Also, I've heard that D-type OD's are smaller and you can get away with some 'adjustment' of the existing tunnel and not need the massive chop-n-replace procedure.
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I'm JohnMc, and I approved this message.