I have read Bob Silvius (is he Sil that post here)

Yeah, the same idiot. ;-)

The dimensions I gave in that article (To make the M41 fit, I cut out a chunk of tunnel 8-1/4" long starting 2-1/4" aft of the shifter hole and 1-1/2" into the floor boards on either side. I also cut the floor of the remainder of the tunnel back to where the emergency brake bracket attaches to it.) are still pretty much right. You could probably get away with cutting 1/4" less into either side without having the "football" rapping against the tunnel wall, but I wouldn't do it. It sometimes helps to be able to wriggle your fingers into that space to retrieve a wire or some such.

Paul, as usual, went about the whole thing a lot more professionally and neatly than I did by freeing the tunnel section right at the fire wall and reattaching it there once he had made the modification. I love the job he did, but you know what they say about cat skinning.

There is no reason why you couldn't weld the replacement piece into place. I used screws because, at that time, I didn't have a welder that could handle the sheet metal without burning big holes in it. I could have riveted it, but I figured that screws would be just about as strong and would give the added advantage of removeability which might come in handy if I ever wanted to lower the OD without pulling the engine. (The rope trick.) As it turned out, I did remove it once to fiddle with the 4th gear contact switch.
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"62 PV544 (B20, M41), "71 142E, "93 240 Classic Wagon.