Cheapest way to change ratios on an 8-3/4, and what ratio to choose?

How tall are your tires? That plays a big part in the "final drive ratio". That cam may idle kinda rough but it's not that big at all in a 408. Just 4 degrees bigger than what is in my 408 and mine is a stump puller. With decent 275-60/15 drag radials mine will break em loose in 2nd gear at a 20-25 mph punch and leave 30-40 foot black marks on the road. And that's with 3.55 gears. I'd be willing to be that unless you have some really tall tires you could put some 3.23's in that'll drop the cruise rpm a whole bunch. As mentioned fuel mileage will never be great but for example mine with just a slightly smaller cam and slightly less compression (9.8 to 1) averages 13.5 mpg. When I keep my foot out of it it'll pull down 15 mpg as long as I don't go over 60. 3.23 geared chunks are plentiful and usually fairly cheap even if they have a sure grip. I'd bet 3.23's will help some on fuel mileage, maybe 2 mpg. Not a huge increase but every little bit helps