"Upgrading" as you call it to a 3.73 axle does give you a top, 4th, gear about the same as fitting an overdrive, 5th, gear. Problem is, the other 3 gears then in effect all jump up one giving you a 1st gear that's about the same as second is now. This doesn't work unless you're prepared to slip the clutch a lot getting moving and you also have LOTS more power to enable you to make the top gear usable at all times rather than having it as a cruising gear like the overdrive. You'd still find yourself running around in 3rd gear most of the time. Go M41. Yes it does need a bit of floor modification but it's well documented and works well.