Its cheapest and easiest to go with what you presently have. If your OHV goes, find another one and bolt it in. No modifications needed and you know what to expect.
Theres presently just about as much for the SOHC in aftermarket as there is for the OHV. The ' power ' differences in the engines Are noticible on the top end, but down low or for offroading, the OHV would be the better engine ( IMO ). Pushrods beat overhead cams for torque.
A 302 can have just as good of gas efficiency as both of the v6 engines your speaking of. Some have an average of 20mpg on reports. I average about 12-15 with an SOHC thats been a bit modified. Others here have stated 15 up to 20. Different areas, elevations, air density.. many factors play in fuel efficiency, no 2 engines will have the exact same.
Once you mentioned ' econemy ', changing to anything different outside of a 4 banger means your not about a swap to something different as it will cost you more to swap to something else.. your looking to SAVE money, not spend more. for as many miles as you would have to drive to make up the cost difference.. it just wouldn't be worth it.