How difficult is doing this yourself ?

It does not look too hard, just time consuming.

I have 3.55 and i wanna bump up to 4:10, possibly 4:56

 

You don't want to do gears your self unless you are very experienced and have all the tools. On a ford 9 or a Toyota rear axle maybe. But if you make one mistake setting up the gears it's going to be a 4 digit mistake.