I'm reading about " Loading the engine"
then using prop pitch to adjust air speed.
That's my usual M.O. but This engine is
something completely different.
Part of the puzzle might be the 69 oz
SV-11 is too much for the .67.
To the last, I would be very surprised if that was true. All of these large stunt engines are extremely "powerful" compared to older engines and the rules for weights went out the window in about 1988 when people figured out how to take advantage of them. If anything the airplane is smaller than ideal for the engine, but it will be OK at 69 ounces.
Bearing in mind that I don't have a 67, just helped other people with them, I would suggest starting with something like 13-5 or 14-5 two-blade, or a 12.5 or 13-5 three blade, and then adjust the speed to satisfactory. If it is running way down in a 4-stroke, reduce the pitch about .2 at a time until it is running up in a fast 4 and maybe breaking in the hard corners. If it's breaking too often, increase the pitch .2 at a time until it's in a 4 most of the time. If it winds up outside the range of 4.5 to 5.5", change the venturi either larger or smaller a few drill sizes and try again. Choose the diameter based on the airplane performance - one of the reasons I haven't tried pursuing engines like the 67, 76, or other muffler engines is that you end up with too much diameter to make the corners clean.
Note that you could take the same advice as above and target 4.5" of pitch, or trade off the diameter VS the pitch, and you probably should. In any case you will get a flyable system, the problem is that there are many flyable systems and you have to decide which of the many possible options is best overall. This is also why you should discount any sort of reviews, evaluation, advise from the guys who go out and fly 6 different engines in an afternoon, then pronounce judgement. Just the first step as described above might take 5 full flying sessions, and once you have 4-5 different approaches, it might take a season or two to decide which one is the best for your airplane and varying weather conditions. I got 90% of my current RO-Jett setup in about 3 months of pretty dedicated work, but we had 15 years of pre-work on other engines between three nationally competitive modelers to draw on, and I am still learning things after having tried almost everything, and come back the same setup time after time over, for 12 years.
Brett