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.