If you're sticking with the .40LA, I'd change props, lower nitro, add a head gasket, shift the CG aft, move the LO's, and change to longer .015" lines (62' eye to eye, maybe)...all, before I went to a smaller venturi. Runaway is normally heat related...not enough total running time, low oil content, too much prop, too restrictive a muffler, etc. Read Randy's pinned posts. Note that a less restrictive muffler = bigger venturi, etc. A bigger venturi, running more fuel through the engine can make it run cooler and more stable with the same load. Burning more fuel is often better.
Pete Ferguson had a .40LA on a Brodak Profile Cardinal that didn't run well. Put on the magic 11 x 4.5 TT Cyclone prop, a head gasket (I think) and a Randy Aero CNC tube muffler (to move the CG aft), 10-22 Powermaster, with a .157" spraybar through the stock venturi, and it was quite nice. Note that early .40LA's had boost ports, but later ones do not. Which is yours?
Steve