Better to use correct size for handling and tire wear.
Stock rims = stock tire
3.25-19
4.0-18
Those 120/90 I have tested on stock rear rim got bad handling rather soon due to the quicker flattened profile. Bike transformed to a snake on lightly curved roads.
4.0-18 kept most of the handling until rubber had disappeared enough for a change.
Huge difference.
CB750 had a bad reputation about bad handling back in late 70's early 80's.
I'm sure the owners were to blame that changed to wider tires for the look only.
I know, I was in it too. Last tire late 80's was 130 wide on stock 2.15 rear rim. Tire was really flat with worn profile as a box.
Details I and my biker friends heard from the older boring men we did not listened at. Important with cooler look.
If you sum up the cost for good old hubs, change bearings, retainers, seals. Remove old bearings, clean inside of hubs from all nasty old stinking grease inside that sit there with stock bearings.
Polish hubs, mount bearings etc.
New rims with spokes to be laced correctly.
New tubes and tires. Mount them.
That's not cheap either.
I will not change rims on my existing good wheels. I bought old hubs I restored.
Rear hub a chance when brake drum/lining can have horrible wear or cracked, not usable.
I was lucky to find a low priced rear hub that looked as sh!t. Brake liner was really rusty. Holes for spokes had no wear at all as happen by use (oval holes). So I knew it was a low miler.
Sanded off the rust and it look nice. No wear edge inside outside of brake shoes either.
The bike that hub sat on cannot been used much.