72 Dart Hemi swap

The driver side header is a tight fit on my 71 Demon. I went with the 1 3/4" Tti headers on the 5.7 and I actually have to remove the torsion bar to get that header in/out. The passenger side fits great with no issues. I have a built 727 and was going to do an overdrive swap but didn't want to mess with the tunnel mods. From what I understand, the Gear Vendors only require a little hammer work in the tunnel for clearance. I would like to do a GV but they aren't cheap and the driveline will need to be shortened. Currently I have 4.10 gears in the car with a 26" tall tire and the rpm's are 3400 @ 65 mph. I plan on going to a 285/70-15 tire which will drop the rpm to about 2900 and with a GV it would drop to about 2300 rpm @ 65.