Final drive ratio question

[Turner & Turner <turner@turner.se>]

Looking in my BL parts book, I note that diffs come in two ratios: 3.7
for the carb cars and 3.4 for the PIs.

My questions:

1. Why is this?

2. After much consideration, I'm going to rebuild my TR6 with a J-type
overdrive and high-torque / fast road setup. Ultimately, I'm more
interested in acceleration than top end, so what diff ratio suits me
best?

Adam Turner
'74 TR6