As I have been told, the rule of thumb is to keep 1st Gear X Rear Gear between 10 and 11 (at least for street cars). Higher than 11 and 1st gear is pretty useless, lower and you don’t have enough gear, I guess causing tuning and clutch issues. I suppose this also assumes normal height street tires, not giant slicks. Super tall tires would likely alter the decision process – and I don’t know drag racing needs enough to comment. So based on that, here is the range you might look at:
2.66 x 4.56 = 12.1
2.66 x 4.11 = 10.9 <--in range
2.66 x 3.90 = 10.4 <--in range
2.66 x 3.73 = 9.9 <--in range
2.66 x 3.42 = 9.1
Cruising RPM, based on .50 6th (I am assuming 4th gen T56), Tire OD 25.8 (275/40/17)
GEAR 55m 65m 75mph
4.11 1471 1739 2007
3.90 1396 1650 1904
3.73 1335 1578 1821 (You‘d barely use this gear on the street
)
The 4.11s or 3.90s look tasty. And it seems some experience with T56 4th gens around here support this. Just an opinion based on what I have been told over the years.