If you choose a 5.9 or a 4-53 the Roadranger bolts on with the proper flywheel housing.
Soon I should have some more info on the RTO6610 that is for sale, if I remember correctly it's the cheapest one I know of for sale right now. And I am continually looking for RTO 6xxx transmissions.
Are you sure you want a Detroit?
Have you driven a Roadranger? I had not until I bought my last truck, and it took me about 4,000 miles, from coast to coast before I felt comfortable shifting it, (I bought it in CA and drove it home). Those first few days were interesting in the traffic in and around San Fransisco.
Grigg