$20k to do a motor 5 years ago before the 6.4's came out, $20k to do a 7.3 now when you can buy a 6.4 for that now, and get at least a 5 year newer truck, and make 550hp with a tuner...
Trust me, I loved my 7.3, but throwing that much money into an old truck to make 5-600hp is stupid. I can...