I just penned this for the 2.5" CE thread, but copied it here for relevance.
I'll add pics to this post this evening, at work for another 7-8 hours.
To do the cooler swap correctly I would do a complete disassembly and put it together like they do on the assembly line. You could cut a few corners but you take the chance of it ending up being twisted somewhat and risk damaging the plastics due to the heat needed to soften the glue. Not to mention it will be a pain to remove the old glue with everything else in the way, and you want good well bonded joints to retain the chassis rigidity. With a few days (long weekend, it is doable). Another reason to pull her down is you need to unglue quite a bit of joints to get it out and you need to clean all the old glue off to re-assemble. To make this easier I would; separate the tunnel from the bulkhead. Remove the lower bulkhead side pieces. Clean the old glue off of all exposed joints. Change the extruded close off for the cooler, reline up the tunnel verifying both are level and square. Re-apply adhesive and re-glue fasten lower bulkhead pieces. Let cure and continue re-assembly. For the average hobby mechanic I would allow a day minimum for disassembly and another for re-assembly. Have Fun!
I would bet it is actually spelled out step by step in the service manual as well, if you have access to one. I have not looked for that exact replacement scenario but it covers most every glued parts replacement in detail. IE; where to glue and how much, fastener torque values, etc. with photos. Probably well worth the modest manual fee, and covers everything other aspect of the sled as well.
Side view of the extruded front close off with the extruded cooler in the back ground, as you can see the bolt bosses are the highest points on either one.
As further shown in the above three photos with the straight edge the highest points in common are indeed the bolt bosses.
This photo shows the only exception in height intrusion into the tunnel, this is where they cut the coolant channel thru the bolt boss and welded this patch over it. The weld bead is the only exception, and it is less than an 1/16" and about 1/2"-5/8" wide in 2 spots. Essentially negligible in the grand scheme of things. If I were making this part, I would have saved an extra milling machine setup and machined the coolant channel from the front side (as they are already milling lightening channels in the bolt bosses from that side, as shown in the below photo) and then put the patch inside the engine compartment instead.
And a view of the Engine compartment side, again while the cooler takes up a bit more of the interior room, it does not extend past the bolt bosses, other than the hose barbs.
