To bend the frame narrower, you will need to squeeze the legs together in one spot and spread them apart in another, so think about how you would make the frame legs into an double curve (S shape).
Keep in mind that you have to slightly overbend the frame to make it stay, but don’t go so far that you have to bend it back.
Tools needed: Two sliding bar clamps with rubber clamp protectors, two blocks ~100mm wide. Blocks should be made of hard wood, metal, or plastic so it won’t deform. The block will need to be ~100mm, you need to determine the exact width needed by measuring your frame, but it needs to hold you frame at 100mm while you bend the frame. Adding a little curve to the block may help prevent tube kinking.
Start by marking your frame for the lower bend, say 1/3 the distance from the bearing holders to the existing leg bend, place a block at that point between the legs and tape it in place. This bend will take place BELOW the block to restore bearing cap alignment.
For the second bend, place a block in the existing leg bend, for this bend you will not be adding a new bend, but increasing the existing bend; this makes it a little easier as the bend is already there and should make it easier to add a little more curve.
Once the blocks are in place, try to visualize the two bends:
The upper bend “narrows” the frame
The lower bend “restores” bearing alignment
You can block and bend the upper and lower frame at the same time, working both sets of clamps simultaneously, just be carefull the clamps and blocks don’t slip. You don’t have to use a hub to maintain the bearing holder width, but you will need the hub to determine final alignment.
The key issue is to make the two legs bend equally, so you don’t end up with a crooked frame. If one leg bends more than the other, you may need to modify you blocking technique to hold one leg steady as you bend the other to match.
Don’t sweat it, it’s not that hard.
Once you get the frame fit to your new hub, build the wheel and go party!
The frame will probably be a little crooked, so you’ll end up having to align the rim to center, so build your wheel facing the direction you plan to run it.