Roadrangers don't always come on detroits, so don't think that with a Detroit you need a Roadranger...
You could use either a small roadranger or an Allison on any of your listed engine choices.
about actual fit and modifications.
The 4Bt shouldn't require any or very minimal sheetmetal work.
The 6BT will require huge amounts of firewall work.
The 4-53T requires some firewall work, don't need any extra length, just need more width to the existing indentation.
I used a 3" body lift on my truck to make some of the clearance problems work out.
Grigg
