Hi Roger
I have 2 Stalker .40RE's set up in different sized, and styled planes. One is serving in a kit built Vector 40. It handles the small plane with good authority. I use FAI fuel, an 11 X 6 2 blade prop, and launch about 86 to 88 k rpm.
The other plane is considerably larger, 620 sq inches, but the weight is in the mid 40 ounce region. I use a 10 X 6 3 blade, and again launch at about the same rpm. Here in Salt Lake City, where the altitude is over 4600 feet ASL, the engine has the power needed, but tends to require a bit higher launch speed on the longish lines I'm using. I expect that shortening the lines a foor or two, will make the wingover less of a free flight event. Everywhere else the line tension is fine. The same setup at lower altitudes has worked very well. The runs are consistent, and reliable.
I would consider using the 40RE in most cases, for a 40 sized plane, unless it was a real porker. In that case, consider the .51RE, a sweetheart of an engine.