The inners should be fine.

I once squeezed some powerflex outers through the standard bush outer collet, but they were too tight and mushroomed out of the end

I think your best bet is to weld on an extra short piece of tube where the wishbone stops (As you suggest) but maybe ever so slightly smaller than the diameter of the hole in the wishbone. (So that the bush is tight in the hole, but not so tight as it is distorted.)

If you weld these bits on, they should stop 3mm or so short of the stub axle assembly when it is all bolted up tight. Dont make them too long so that they are pressing on the stub axle assy before the inner sleeve is tight (If you know what i mean).

Good luck

PS i binned my Powerflex ones and got some others that came with outer collets, but i cant remember the manufacturer