Am I right that if a MTB threaded bb type of cranks, hollowtech, box or whatever, that are listed as 68/73mm, they can me made to fit on road frames with a resultant half decent chain line with the use of a spacer or two?

They can but it can be difficult to get them to ever work perfectly. I’ve done it in the past to get some relatively cheap 180 cranks on a road bike but it was never quite right.

I’d expect chain line to be out and obvs road front mech would be out

Depends on the rear axle spacing of the road bike.

If its a disc frame with 135 then chain line will be OK, just like mtb so BB spacer either side (2 on drive side for shimano)

If a 130 then you’ll need to put the 2x bb spacers on the non drive side (and one drive side for shimano) to get good chainline. This will offset the cranks in the frame a little.

If you are running a single ring on a double mtb crank you can just space the chainring in 2.5mm instead.

Cheers for the replies. It will be for a disc frame, 1×10 set up.

If a 130 then you’ll need to put the 2x bb spacers on the non drive side (and one drive side for shimano) to get good chainline. This will offset the cranks in the frame a little.

This’ll sort the chainline but won’t this give him a nasty q factor ?

Cheers for the replies. It will be for a disc frame, 1×10 set up

Surely only difference from doing this on an MTB is that the chainstay is shorter, which slightly exagerates the angles