Yes -change the whole banjo but you'll probably have to use the Wasp flange for the Wasp - it's a deeper dished
flange (so crack its nut before (Wasp) banjo removal).
Ditto if the Florian has Bellett diff gears - something us BCCV Treasurers usually did - Florians being 4.1 with Belletts usually ~3.7 ratio compared to Wasp and common KBs & Rodeo & Jackaroos being the standard commercial 4.55. (I was to get a KB ~5.25 ratio for rallying until i discovered the 400Nm G200W...)
IE - banjos are interchangeable and axles have the same diameter and splines until the 4ZE1 engine.
Sometimes a higher diff ratio increases speed - that's a function of the engine's torque characteristic.
Wasps had the 3 bearing G150 with iron head and i don't recall their torque curve.
I got the impression std G150 Wasps were fairly gutless compared to my G161-OHV Wasp.
[ IMO the old 3 bearing G150s were "zippier" than 5-brg G150s but 5brgs had more engine torque/power and that should mean higher top speed assuming a suitable gear/diff ratio. I typically use the (early Gemini) 0.782 5th else common 0.855 - i haven't used a Bellett or Florian 4 speed in over 2 decades. ]
My Wasp had a standard spec Florian G161 engine except for Gemini conrods & pistons (obviously) and a slightly torquier cam, but even being a mere Nikki carbied 1600cc with a 0.782 fifth I still wanted a 6th gear... And that was before its later standard G-series electronic dizzy (ie the "RG" dizzy) though i'm unsure of its impact.
The RG imo being the best ignition you can have except for CDI etc. Ironically (imo) the RG is also the cheapest (DIY etc), and of course uses all Isuzu-OEM & common parts for those strict Marque
collectors or "Modified" category concourses (or were in-Marque mods better in the st'd category...?).
Though some non-CDI aftermarket ignitions might pip the RG if they have super
igCoils, just substitute the RG's blue ND for a Bosch MEC etc. My Wasp ran an MEC 723 (120mJ) for a while but i changed back because i was too lazy to fit the passives to stabilise its (Jackaroo) tacho. Not that i tested if the RG's ignitor could supply full MEC current, but the RG ignitor is self protecting. ]
Sorry, i digressed...
I was to change my Wasp's LSD to to 3.9 using a Piazza LSD else Piazza crown & pinion in a pre-4ZE1 Jackaroo/Rodeo LSD but i got rid of her before that.
The point being that i was ready to try a 3.9 in the Wasp with mere G161 pushrod engine though i think 4.1 would have been more appropriate - especially with my 15" rims (195/60?? tyres) - but i wanted the LSD.
But if you have the standard Wasp 3 bearing G150... with early carby (& generator, & bendix... oh, it's all coming back to me!) I'd suspect your choice of 4.1 Florian banjo would be the lowest you would go for top speed.
BTW - don't confuse interchanging Florian & Bellett crown and pinions and then Florian BANJOS with Wasps, LUVs, KBs, Jacks, Rodeos etc. Florians & Belletts are 8 bolt crowns. Later (non-Aussie) Florians & most Isuzus until the 4ZE1 are 10 bolt. (Geminis & early Piazzas are usually Salisbury types.)
And obviously the non-existent (non-B-type) Bellett banjo
won't interchange with banjo'd Isuzus (Florians, KBs, Piazzas etc).
I've seen someone interpret the above info as meaning Bellett & Florians diffs are interchangeable, hence Jackaroos fit Belletts etc etc.
FYI - With my 300Nm@2kRPM G200W Florian i was keen for a 2:1
ratio diff... imagine cruising a lazy 150kmh at 2,000 RPM... (Though that engine was tuned for peak torque/economy at highway cruising speeds - namely ~380Nm@3kRPM. I did begin torque/power versus high speed resistance calcs for a top speed & gearing estimate but i never completed them. It was a standard G200W with the usual twin DCOE45/48 & 2:1 exhaust tune - i didn't do the 170kW cam grind.)
If only those ~3.4 ratio import Piazzas had banjo diffs!
But there are always Getrag boxes...
Damn. Another TL:DR.