mscoot said:
What about knocking a dent in the frame itself [to gain clearance]? Is it the frames' welds that are too thick or is it the tubing? If it's the tubing perhaps a small dent would do?
You know what? To me it looks like you could get away with two small, well placed dents on the underside of each chainstay?
Yes, brilliant solution I've used myself on non-bicycle applications, so I'm annoyed with myself I didn't think of it!

... That said, here's a tip on how I've done this before that can help here:
1) location: Mark each spot where a dent is needed with ink from a felt-tip pen - writes best on a painted frame, and will wipe away afterward, so don't touch it until you don't need it any more. Put your mark on the side, 90 degrees around the tube from where you want the dent, in the center of the spot you want dented.
2) "dolly" selection - diameter: Your dent's shape should be set by an intermediate tool usually called a dolly. For this, select a steel bolt of sufficient diameter as to represent the radius of curve you want for your dent, and don't go too small! A large radius dent is safer for the part being dented to not fail, so go bigger rather than smaller, but not so big as you have to move a lot more metal - find a balance point. For example, if M8 is the minimum, maybe go M10, and consider whether M12 would be too big or not.
3) hammer selection - mass: I've used many different kinds of hammers to shape metal and in a case like this you want the heaviest one on hand that you can swing with some accuracy. The mass means you don't have to swing fast and hard and so, swinging slower and not so hard makes it easier to get the blows right where you want them, but if it's too heavy you have that problem because it's too big...
4) positioning the dolly: You can free-hand the positioning as you go, but sometimes I will use rubber bands or tape to position the dolly, you just have to check that it stays in position as you do your work. In any event, you want it exactly perpendicular to the place you need "raised" to gain clearance, and, of course, exactly at the spot.
5) applying force: You have a bit of a dilemma in that metals work-harden as you re-shape them - they literally become stiffer - and more brittle and that works against you, and on the other hand making your dent all at one time means you'd better get it in the right spot and right depth in one blow. So, I go for the middle ground. Since you don't know just how easily the tube will give way, start too light and add force until you see it start to yield, then ramp up the blows in force some and make sure every blow is putting force exactly where the previous was done - "micro-denting" will work-harden the piece considerably and you really don't want that, so even if you don't start in the exactly perfect position, it might be better to keep going with it slightly off than to try and correct it. The one exception is the rotational position, which you can alter with no ill effects - it's just geometry!
Good luck.