I have heard of people getting 18 to 20 MPG with the 4 speed and I-6....I am pretty sure the 4 speed is the NP 435 which is as solid as they come in theses trucks and used both for the V8's as well as the I-6...if i am wrong on that someone will chime in soon i am sure.

As for Highway speeds that will depend on what your ring and pinion gearing is....I know that the 300 six is an awesome engine and has great torque but that is inherent with most inline engines.
I personally cannot advise as to how it will do towing a 22 foot sailboat but again i shall defer that to some of the guys that own trucks with the inline in them.

I do know that the 78 i had had A/C in it with the 300 six and it use to blow meat locker cold...(wish i had never sold that truck).

Anyway be patient some of the others will chime in soon with their knowledge.