I'm not sure it's going to actually be "way less cutting" when you're done, especially with any kind of significant upper crossmember. I know I already posted a picture of my finished tunnel for my T56, but take a look at this one from when it was all original sheet metal...
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Yes, it's definitely more cutting, not arguing that. But that's with the upper crossmember in place. Also keep in mine I was going from a 4-speed to the T56, so some of the welds on the driver's side were welding the sheet metal I removed for the 4 speed conversion back into place. You can see on the passenger side I didn't have any welds along the base of the tunnel, just at the crossmember where I put in some relief cuts. I made the top panel rectangular just for ease of fabrication, so on the finished product it looks like the two halves of the tunnel were further apart than they actually were. My starting point looked like this. I saved the sheet metal I cut out for the 4 speed conversion, so when I did the T56 I just put it back
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This is how I finished the rear section, and I did this because of how the transmission was installing with the engine already in place. With the Milodon road race pan on my 340 and the spool mount K if the engine comes out the top it has to do so without the flywheel, it literally can only go straight up. So the transmission has to be a completely separate deal unless you drop the K and take everything out the bottom. So the opening and removable panel behind the shifter is so the transmission can be at the right height and slide straight forward into the bell housing, which made the rear tunnel a bit larger. Some of the new metal was also because of the previous 4 speed install. But I wanted to be able to drop the transmission independent of the bell housing, because it's much easier than pulling the assembly with the Quicktime bell attached because of its size and weight, not to mention the hydraulic clutch hardware. Same kinda idea for the plugs, the rear one lets the harness for my O2 sensor through as well as the reverse lockout wiring, the forward one on the driver's side is for hydraulic clutch access, and there's one on the passenger side to make one of the bell housing bolts easier to get to. Could you do it without the removable panel and plugs? Yup, sure could. But then you'd be tied into dropping the entire assemble out the bottom, instead of being able to separate the transmission from the bell. More work for sure, but more options for repair if it's necessary.
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So bottom line what I'm saying is that with the upper crossmember in place, that small split down the middle of the tunnel is going to get larger because the upper crossmember is going to be higher than the transmission. You may also find that with the upper crossmember in place you may also need to make those relief cuts larger to shape the tunnel. And that means it's not going to be all that different from I did for the T56. I'm sure it will still be a bit less cutting, but I think it will be more than you've done so far.
