OK I think I can weigh in on this as I can compare my old '68 Sprite Bugsy I with a 4.22 Pumpkin to driving Bill Mayer's 60FROG with a 3.9 rear and a 5-Speed.
1) First and foremost, Synchro First Gear. Year, a usable First Gear that transforms the car.
2) First Gear with a Datsun 5 Speed is a lower gear ratio that a Ribcase 4 Speed. You will find that you will run out of first gear faster and need to shift sooner without the 3.9 Pumpkin
3) I noticed very little perceived performance difference in 1st Gear in fact I think it was enhanced with better matched tranny and read end ratios.
4) The real beauty is simply being able to stay with Modern Traffic. Bugsy I in spite of rebuilt shocks, rebuilt front end, all new bushings was really pushing to get over 60, 65 a bit scarier, and 70 engine absolutely screaming and it felt uncomfortable to go there even on a smooth expressway you were fighting to maintain any sort of speed. Now that engine was quite worn and definitely needed a rebuild but even at that 60-65 absolutely tops.
5) 60FROG - Engine is a delight thanks to Scott McDonald's build efforts and Bill M's wrenching abilities, but 60FROG is able to cruise effortlessly at 70 MPh when needed, I ran with traffic on the I-15coming back into San Diego turning a steady 80 Mph and even popped it up to 85 Mph without an issue or a hesitation. The car felt absolutely Rock Solid and I was comfortable even at 85. And yes I was being passed by pickups with big tires doing 90. Those you heard and felt coming before they would overtake you. Mirror visibility at that speed is not good but stay in your lane, flow with the traffic and just fine.
6) Canyon carving - Bill Mayer introduced me to some great roads to the West of San Diego. Lot's of fast turns and high speed turns and the 5 speed ration were just perfect. Those were mainly run in 3rd and 4th gear as there is a performance drop off when you shift into 5th. It is an overdrive. But running in 3rd and 4th in those twisties was a whole lot of fun.
Will I install a 5 Speed and a 3.9. You bet, I bought Gerard's Kit several years ago and I found a 3.9 from a lister many of you know from the Velocity Channel, Mike Joy, that is on it's way to me next week. If you want a 3.9 just talk to the folks on mgexp board and you will likely find one. 3 people responded to my inquiry over there looking for one. The 3.9 IIRC is simply a drop in, replace gasket, add a set of EN17 Axles which I acquired as well and replace the Radius Arms Bushings and Spring Bushings and you have transformed a BE Rear end.
To go even better I found a rebuilt 1275 +.040 = 1,326 from another forum member that will go into Bugsy IV as well. It will be here next week. I will drive Bugsy IV for the next few weeks until it gets too cold and then start with the rebuild project and mods I am planning on making to Bugsy IV. Hoping by late Spring I will be able to drop in the engine and tranny. I will drive ugly all Summer and Fall and pull him apart for Paint over the winter, and hopefully reassemble and thrashing in Late Spring next year. Going to be fun,
Do you want to add the 3.90 Pumpkin. An absolute YES. The combination of the 5 -Speed and lower Rear End ratio will transform your LBC.