Torque Monster !

The thing is what are you looking for more pulling power at crusin speed or fuel mileage ??

It’s hard to gain both one gonna sacrifice the other.

Say your crusin rpm is 2000 rpm and the 440 say is making 80 hp and consuming 255 cfm of air and fuel.

To make 80 hp the 440 would have to make 210 lbs ft at 2000 rpm also. Now say you stroked it to 500 cid it would basically make 80 hp and 233 lbs ft at 1760 rpm and also consume 255 cfm of air and fuel. So you would spend a lot of money to drop 240 rpm and probably have similar power, gas mileage and wear and tear. You could spin either engine higher for more power at the cost of mpg or do the opposite.

Adding a OD to the 440 would probably be the best bet unless the 440 is having a hard time pulling it now. Might have to add deeper gears depending on how low the rpm drops.

As for torque power and mpg would be better off making the 440 more efficient and gain all 3.
Which is hard and expensive and even with the miles you put on a motor home hard to recoup. But CR, thermal efficiency, lower intake and exhaust restrictions etc.. should be where to look.