3:42 is NOT a pulling gear... especially for a gasser and not if you will be getting anywhere near the rated towing capacity. It can be utilized in a diesel application with a 5/6 speed, but mainly due to the diesels ability to produce max torque at low RPM. That being said, IF you only pull occasionally, and run at lower weights...it might be a good trade-off for the increased fuel economy when dead-heading..
Personally, I would not buy a gasser with less than 4:10 gearing.... Granted, i would give up fuel economy, but I would be absolutey steamed if I were to run out of pulling power!!!. My personaly experience is that what fuel economy I could gain by running 3:56 or even 3:73 is negated when the trans. is constantly coming back to 4th or even 3rd gear when the 4:10 DOESN'T backshift... Not to mention the incresed wear of full load backshifting on the trans...