I've already posted this in the bicycle mechanics forum, not sure if that's kopacetic, so apologies if it's not.

Anyway...

I bought a mid-70s Motobecane Grand Jubile frame that the previous owner had cold set the rear dropout from 120mm (I imagine) to 130mm.

I had wheels built around 130mm Grand Bois hubs that will accept either Shimano or Campy cassette. Trying to save funds after that expense, I found an unused Shimano Hyperglide six-speed (14 to 28) cassette in my parts bin.

Bike has the original Huret front and rear derailleurs, and I've read a 28 tooth cog is about the largest the Jubilee can handle. Cranks are 52 and 42 tooth. I don't mind keeping it more or less original gearing, but my question is: since the original chain line would have been for 120 mm hub, what problems am I likely to run into with an extra 10 mm hub width? Would I need spacer of some sort between the cassette and the hub? Or is my best option to run an eight, nine or 10-speed cassette, even though that would be beyond a half-century-old French drivetrain's capacity?

Thanks in advance!

Forgot to mention, I put in a Velo Orange bottom bracket in 116 mm length.

Others more knowledgeable or experienced will pipe up.
First, it isn't 10mm extra, it is 5. you need to split it for both sides.
Likely the hub width isn't much different as rim dishing will keep the block closer to the chain line. The difference will be small enough not to matter too much.
Run whatever count of sprockets you want. I would space the block as close to the hub flange as the RD will allow without getting into the spokes in the lowest gear. Match the chain to the number of sprockets on the block.

What was the original length? 120 and 130 have the same chainline.

Right. Math. There's a reason I always re-work the title of that Ben Affleck/Morgan Freeman movie to refer to myself: the Fear of All Sums.

I don’t know if that derailer can handle a cog that wide. You could run into clearance at the dropout inside on the chainstay

You won't know till you try, but one issue you could run into, is that when you are on the small chain ring and smallest cog, the chain could rub on the side of the large chain ring.
But first things first- measure the existing chain line.

A few thoughts:

First a Motobecane almost certainly has Swiss threads are you using the threadless VO BB?

Any VO BB will have a JIS tapered spindle, what crankset are you using? Wether Japanese or French your bicycles stock crank was likely intended for an ISO tapered axle. What this means is that your chainline is likely further outboard than stock as is.

My recommendation is to set it up the way you want it and experiment. The combination of a modern chain and friction shifting should be very accommodating.

Also your use of the gears can be a big factor. For example if only you only seldom use the low gears than having the front rings further out may not be a very big deal, or if you seldom ever use the 2 or 3 highest rear cogs than having your front chainrings inboard a little may result in a better chainline in the gear combinations you frequently ride in.

https://www.sheldonbrown.com/bbtaper.html

What is the crankset..... BB spindle length is dependent on the crankset