I never thought about any warpage factors back in the day.. it was all about "build and fly"... by the seat of the pants more often then not. Even if it might have flexed I never knew it. I put these planes through a ringer too.
Years and years ago, on one of my old Fun Scale .40's, I did have half of the rear horizontal stab/elevator break off one day though... right up next to the fuselage. That's a vivid memory! I had the plane nose-bleed high and was cranking it like mad.. and then something flew off (the plane was so high I couldn't see what it was..).. so in a slight panic, I throttled back and coasted down... and the plane was somewhat erratic.. then I realized half that hor.stab and elev was gone!.. Fortunately the half that was left there had the pushrod/elevator linkage still intact operating the remaining elevator. Landing was actually uneventful... I searched but never found the parts that came off the plane... So ... moral.. make sure you're rear stabs are secured.. ha. I built a new stab-half/elev and scabbed it in there somehow.. as that wasn't the end of that plane by a long shot.
The only other area that I remember being marginal was that main-spar/landing-gear mount set-up. I remember on one my Mustangs that spar actually split from a less then perfect landing... and it torqued the gear wire, and just split that spar quite a bit, and it originated in the hole the landing-gear wire inserts into that spar/wing, and basically cracked at that slot the LG wire seats in. I remember doing surgery/repair on it, and having to cram epoxy into the crack, and just hoping it would hold... I don't remember it ever coming loose again. I lost both of Mustangs to dumb-thumbs and taking chances... oh what fun though!
Yes, I've got a .91FX sitting here with my DynaFlite FS .60 Mustang. I think the newer DynaFlites do have sheeting on the top/leading edge of the wing...(both the .40 and .60).. which might be to alleviate flex.. not sure. but the early generations like the one you're building are a bit different in many ways... but still have a very similar outer shape.
I like you're spar idea... and it just sitting there in your pic it just looks right too. So carry on... and have a great day!