I'd develop a system of straight splints, wood or metal - at least on 4 sides (wrapped in platic wrap or wax paper if they might otherwise end up bonding to the mast) that extended several feet on either side of the joint.  Then clamp them on and use your eyeball to get everything straight, and supports to keep things that way.  Your eyeball will get things strainghter than anything else.  Remember that everything will get much more slippery and harder to keep lined up once the glue is applied.

And although MAS or whatever epoxy you have at hand might suffice, I'd suggest some JB Weld, as it is specifically made for jobs like this.  I've had good luck with it on other occassions for reasonably similar jobs.

Probably reallly not much better that the epoxy fix - and certainly more effort and expense - would be to allow an aluminum welder to drill holes in the outer metal pieces (I'm assuming that is the mast pieces, not the connectors).  The welder would decide where to drill and how many holes (welds) are necessary.  Then you slide the assembly together and the welder makes "rosette" welds at the site of the holes.  Done well, these can even be filed smooth and thus become almost invisible repairs.  

In the way-back-when on my dad's float plane we bent (creased) a float cross-member spreader strut.  This was repaired by finding a similar but smaller diameter airfoil shaped strut (I don't know how Dad found that, but it sure fit perfectly), sliding it in the ends a foot or two where we cut off each side of the original strut, and doing the rosette welds, plus a weld where the two halves of the original sturt re-joined.  Filed, and finished with some 400 grit, the repair became almost invisible, and was OK'd by FAA.