Chevy 305 mated to a T5 tranny in a cj5

[The Phoenix]

My father is looking to put either a t150 or a t5 transmision to a basically stock 305 engine. Just for around the town type of use. My question is the T5 will give him an overdrive gear for better gas milage, but will it be able to handle the stress or is the milage gained by the overdrive gear not worth the reliability of the heavy duty t150
thanks for your input
Tim