I'm doing a 2.46:1 gear swap on my Scout II Dana 20 as well, but I'm not using ANY Dana 18 parts.
I'm using all Bronco parts, including cutting and splicing the top of the Bronco shift-fork to the bottom of the Scout shift-fork. I already welded up and re-notched the shift rails and have Dana 18 shifters to go with the conversion.
I'm just using the front, rear, and intermediate gears from the Bronco, and half of one of the shift forks. (I think that's right.. I need to look at my pile of parts again). Some grinding to the case will be required. I paused on my conversion to finish my new 345 instead.
64:1 with the wide-box T19 I just put in and my 4.09 gears when I get the new 'case done. Twice as low as I was last summer with the close 4spd and stock 'case (and just a stock 304).