To visualize this: imagine a square; draw a dotted line from the upper left
corner to the lower right
. This dotted line will represent a slant of 45Â° from top to bottom. This will show what the glue joint will look like between the old spar stub and the new spar section that you are replacing.
Then the joint between this replacment piece and the (now tapered) end of the remaining original spar (running outboard toward the wing tip) will look just like the first joint that you made. It's probably best to locate these two joints in the middle of separate rib bays in order to be able to access them readily.
As I understand it, a shear web would be a piece of sheet wood BRIDGING AN UPPER SPAR AND A LOWER ONE. This may be how your spars are configured, or maybe not. If there is no
lower spar in this location, you'll need to DOUBLE the spar material both fore and aft across the entire rib bay. When all is done, check the lateral balance again, since the added material at the repair may cause a weight bias on that side.