I would tie top of spar to leading edge with a L angle. That gives allot of strength to wing.
Compare a uncovered PA-12 wing with partial wrap leading edge to a full wrap -18.
I think that this is a very important point.
IMHO
With the extended leading edge installed without any other anchoring other then with screws to the ribs; damage to the ribs will eventually occur.
A PA-12 wing is very easily twisted due to the design of the leading edge. This allows it to flex more in response to turbulence then a PA18. Pick up one end of a bare PA12 wing and you will quickly see how easy it can twist and you will also see buckling of the leading edge skins as you do. You will learn to handle it with care.
A PA-18 wing with the majority of the leading edge locked to the spar, is much more rigid. Enough so that the “wash out” needs to be preset when building. This almost creates a type of “box spar” which is highly resistant to torsional or twisting forces.
Extending the leading edge without this support will leave the wing with same ability to flex or twist when in turbulence or heavy loading. This twisting action with the extended metal will cause similar and more pronounced buckling of the skins and over time, could cause the screws or rivets at the cap strip to loosen or tear out completely. I think that this will be even more accelerated if the stitching to the fabric is not present.
My opinion is that locking the leading edge to the spar is vital at a minimum, especially with the heavy gross weights which we often experience.
I also feel that the stitching is well worth the time. The only problem is that the skin for this particular project needs to be loosened from the ribs to drill the holes, which need to overlap the cap strip.
I like the extended leading edge and have it on my 180 cub.
It is installed as an addition to the standard piper skins; which lock to the spar along with stitching.
I have just completed recovering them after 20 plus years in service. There was no looseness or damage observed.