So you have a 2.87 First gear. For years I have used an overall ratio of 10:1 for the First gear, as a calculation goal. 10 / 2.87 = 3.48. In a Dana 60 I'd use a 3.54 gearset. Ford 9" I would use a 3.50. 8 3/4 I'd use a 3.55.

That's just a rule of thumb but it has worked for me.

R.