The 4L60 has a modular bell housing, there is a direct fit 4L60 to nailhead bell housing available. CARs has it, a bit pricy but eliminates the need for an adapter from the nailhead to the 200R4 and the need to cut your tunnel. The 4L60 also has a deeper first gear. I have one in a 65, things line but well but you do need to move the crossmember aft but it's no big deal. Shiftworks has a shift kit that goes well with the GS console. Talk to your tracy guy about beefing up either trany to handle the nailhead toque.
You'll enjoy seeing the rpm drop from 3000+ to 1800 or 1900, you'll ask yourself why you didn't do it sooner.