I understand the diff gears are:
4.30X:1 Grand manual (XL-7 manual)
4.625:1 Base 2.0L manual
So by keeping the 4.625, but fitting the V6 engine and transmission, you already have a geared down GV fit, ratio 1.075.
As you have upgeared by ratio 1.15 with the 31" tyres, the result would be like having a GV with 29" tyres. It seems like lots of guys are content with that.
If you later want to fit the 5.13s, you would have the equivalent of a GV on stock tyres. You can always fit them later, but the cheap V6 fit is a once-in-a-lifetime, isn't it?
Btw, fitting the 5.13s and keeping the 2.0 engine and transmission wouild result in the equivalent of 26" tyres, i.e. undergeared, so your highway cruising revs would be higher than now.