Transmission Conversion

[hexter01]

I have a 68 Dually flatbed with a four speed, (granny gear 1st). I am never going to pull a lot with this truck, the heaviest thing may be the cab from another truck to the media blaster.
I have been considering swapping a 400 Turbo in place of the standard.
Is this common, or, are their more issues than makes it worthwhile?
Top end in this truck is about 60 with the current gearing, and I never use first gear. I realize I may have to change the rear end gearing as well, but I am just looking for basic information here.
Thanks, so much to all!
hexter01