Not only for tire clearance. Once you make more than 30 hp you will find the short Blaster too prone to wheelying when you apply power or on hillclimbs. Too long an extension and it will spin too much on any low traction surface. It is a balance.
I like the idea of tack welding the angle brace before cutting. Good idea!
I cut mine further back to avoid the tube section, which I believe is chromemoly:
Chromemoly does not weld very well. It hardens and cracks at the edge of the weld.
I only went with a 2" extension on this pictured swingarm, my son and I later did 4" extensions.
If you are going to modify a stock swingarm this is the place to do it, because:
1) The weld and extension will have the least bending leverage applied to it.
2) You are welding to 1/4" mild steel, easy to get good quality welds.
3) You will have 4 runs of 4" long weld on the 1/4" plate. A lot of strength there.
4) The shock mount position is unchanged. Strength is maintained, travel increased, rate softened.
5) Added weight is kept to a minimum. 3 pieces of plate are lighter than the tube required.
I used the bearing housing as a jig and tacked the extensions in place.
I welded in short stitches on one side then the other to keep from bending.
Makes an ugly weld but keeps it straight.
I really like your idea of tacking the two sides together and may do it on my next extension.
Finally I added a horizontal 1/8" plate to eliminate side to side flexing:
As mentioned, the short stitch welds are not a thing of beauty.
Properly braced I could do a continuous weld.
We have done several of these and so far the only problem is the X cracks that the stock arm suffers anyway.
Even the 4" is holding up well. I kinda favour a 3" extension as the best compromise. With the 4" I sometimes have trouble transferring weight back in slippery terrain. My KTM Blaster still has the 2" extension and as a trail machine I love the ability to loft the front end over logs even while speeding on wet leaves or slimy hardpacked wet mud.
The only 2wd and smallest displacement in the group. Also the lightest by 200 lbs and quickest.
