Andy,
If you want to use 5C collets you have 3 basic options.
1. Spindle bush/ Nose piece and a draw tube.
You can make a 5C taper nose and mount this inside the spindle bore (the bore is morse taper 4 1/2 bush) or you can made a D1-3 backplate mounted 5C taper nose.
I've seen both for sale although I think the 5C internal taper to 4 1/2 morse external was "home" made and I've only seen one on ebay. Several manufacturars have made a D1-3 mount 5C internal taper widgit. Monarch made such an item for the 10EE lathe and so have other manufacturars. They come up regular on ebay.com. It would be a nice project to make one.
If you want to make one fast tracked you can buy on ebay.de a 5C to 5MT bush and turn it down to 4 1/2 MT. or you can buy a 4 1/2 MT external to 3 MT internal from Boxford or get lucky on the second hand market.
As Nick_G points out if you use a drawbar with 5C collets you can't pass stock through. Suboptimal. Better to make or source a draw tube. Since 5C is so common they come up all the time on ebay.com, unlikely that they are exactly the correct length for the boxford but then owning a lathe you can fix that. I think one regular seller makes them professionally. Maybe they would make one to the exact length you need, or leave the end unhardened.
2. Get a 5C collet chuck with D1-3 mount.
Loads to choose from. Some better than others with a range in price to match. Some have a handwheel (speed / Sjogren chucks) some a regular chuck key.
I just looked to remind myself and my IS11.30 does not have any provision for mounting a collet closer on the far end of the spindle. I have learned that closers are very very nice. If you rule out the closer as part of the solution if it were me I would go with a D1-3 mounted 5C chuck.
3. Buy the 5C accessory kit from Boxford. I doubt you'll get much change from a grand.
Steve
Reason for Edit: Clarification, I was typing my post and meanwhile Bazyle posted. So some repeated content.
Edited By SteveI on 02/11/2016 18:08:23