Singlespeed & Fixed Gear - BB Question - Italian Threading but not 70 mm wide?

I am converting a frame that has a threadless bottom bracket, which I am in the process of threading. But I'm not sure what my options are since my last conversion was far more straightforward (english treaded off the bat). I don't think English/ISO threads will fit, but it is likely that Italian threads will fit (slightly bigger). The thing is, the BB shell width is 68 mm, so the Italian (70mm) would leave a 2mm gap...is that likely to be a problem? Hopefully I can leave the gap on the non drive side so as to not screw up the chainline? Or is it possible to use Italian cups with a English 68 mm bottom bracket cartridge with the proper spindle length? I know I need a 107mm BB for the proper chainline. This BB stuff is getting me pretty confused, so any help sorting it out would be greatly appreciated.

How old is the frame and where was it made? I can't imagine it being too large for ISO threads if the width is 68mm. Unless it's really old, and it was designed for French or Swiss BBs (which are also 68mm width). I don't know if French/Swiss where slightly larger though, they might have been the same size as the ISO.

Anyway, the 2mm would probably be acceptable, but I would go for the ISO first. If it doesn't work out, you can always tap out to the Italian threads.

It's an English-made Viscount, Late 70's. From my crude measurements the ISO won't fit, but I think I'm going to take it to a shop and have it checked out since this is going to be a close call. It looks like the bearings are 35mm OD, English threads are ~34.9mm. Italian is 36mm. It's such a cool frame I'm hoping this wont be a big pain!

Matthew A Brown has a Medici with Italian threads, but a 68 mm shell. Ask him what he did, because I know he's got it up and running just fine.

Have you considered adding a 1mm spacer on the fix cup side?

I am converting a frame that has a threadless bottom bracket, which I am in the process of threading. But I'm not sure what my options are since my last conversion was far more straightforward (english treaded off the bat). I don't think English/ISO threads will fit, but it is likely that Italian threads will fit (slightly bigger). The thing is, the BB shell width is 68 mm, so the Italian (70mm) would leave a 2mm gap...is that likely to be a problem? Hopefully I can leave the gap on the non drive side so as to not screw up the chainline? Or is it possible to use Italian cups with a English 68 mm bottom bracket cartridge with the proper spindle length? I know I need a 107mm BB for the proper chainline. This BB stuff is getting me pretty confused, so any help sorting it out would be greatly appreciated.

I have two bikes formerly belonging to pros. Both of them have Italian BBs, one has 68mm shell spaced out to 70mm by washers. Funny thing is there is one washer on each side of the shell. Usually, you can space out the hub, too if there is a problem with the chainline.

Thanks for the suggestions everyone! After I finish I'll let you know what worked on the build.

1mm spacer on each side, or have you seen this.....

http://bikecult.com/works/parts/bbViscount.html

make sure you get rid of the death fork