I found the electric dizzy from a 60 series and 35" tires netted me the biggest gains in MPG. I can get an honest 17mpg when things are really dialed in. 15mpg is more the norm. Great job, I bet a 4 speed will really help you enjoy the rig more.
My rig... "Maude"
1976 FJ40 with some stuff and some leaks.
1999 UZJ 100 Series bone stock ready to rock AKA:The backup minivan
2012 Toyota Sienna Minivan, fastest Toyota in the stable.
My camping trailer build up thread.
"You 80 guys are just a Sawzall away from nirvana." -Red Chili