[Mike C]

I've done a bunch and it is a skill that can be mastered and completed with some special tools. Realistically, to do it right you will need many hundreds of dollars worth of tools. But I would suggest if you haven't put the effort into doing research and just asking the question I don't think I'd start there.

3.73 would be the stock gear, but since you have the 435 which is not that common an option, I would check the SPID to see if there is an optional gear ratio as well.

The easiest, and cheapest, way to get different gears is to buy a complete rear axle from another truck that has the gear ratio that you want already in it. Then just swap axle housings.

Another way to get the gear ratio down is taller tires. I had 31" on my short bed and it put the speedometer off by 10 miles per hour on the highway so figure you can get a pseudo overdrive that way. Now that the truck is lowered I went to 29" tires and it's only off about 5 mph.