As I mentioned in an earlier post, most builders I've seen (for pa-style aircraft) tend to put an extra cross-brace up there anyway. I did it in the mid-90s, when I built the first fuselage. It was 1" x 0.049" back then as I recall, which is the same size as the other tubes up there. I would need to confirm that though--I don't have the plans right in front of me.
As to strengthening the structure up there, it would be easy enough to do...yes. And Solidworks would allow you to simulate the deflection(s) for a given load(s), and with a couple of mouse clicks you can simply change the weldment profile to be a different diameter and/or wall thickness...and then just re-run the simulation to check on the stresses and deflections. So it's a true apples-to-apples type of comparison. I'm working on reacquainting myself with Solidworks Simulation now, as I haven't used the simulation aspect in a couple of years. And then of course I'll need to create an accurate model--but that's only a few hours worth of work. So it's definitely something that would not be terribly difficult to do.
EDIT: Forgot to address your materials question. To my knowledge (as the pa-14 aircraft were built 1947-1949), they were built with 1025 mild steel tubing. This is the same as the pa-18 were built, per the original drawings (and now the Northland drawings). So going to 4130 would offer a significant increase in yield and ultimate tensile strength, without even changing a single diameter or wall thickness. For instance some quick math shows me that the yield strength of 4130 is nearly 25% greater than 1025 steel; and the ultimate tensile strength differential is even a bit higher yet.
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The 4 place cub is a STC Check with Gabe at Airframes it should come with drawings. If you can do solid works just take the 2x2 plans and widen it out. Stick in seaplane doors for easy in and out and a big baggage in the back like Kirk did that is a easy way to help with the W&B, Lots of ways to skin the cat . Stock AOI will work fine, HOWEVER, I have never heard of anybody that did not like more view over the nose in flight and on final. I am keeping my gear, motor mount, and wing attachments stock so I can find a jig if it gets bent. I am using Jays wing attachment fittings and custom motor mount to change the AOI/thrustline. Lowered the upper rear of the stabilizer on the fuselage to make the angles work (thanks Dan Dufalut for that advice). If you are going to be building a one off frame you can build any angles you want into it because you won't have a jig to help you anyway. Don't lower the rear wing fitting, it puts the cables in the hair of the back seat passenger my buddy found this out with his SQ and the boss was not happy! For better or worse I am lucky to have seen a lot of rebuilds and Experimental mods being done up here, Kirk's hanger is at the end of our runway, Airframes used to be out the front of my hanger, My IA has done the rigging on at least 1/2 of all the winning STOL planes up here. Everyone is always willing to share the little tricks that make it work, the downside is making just "one more change" is like candy to a baby! Just a lolly pop to tempt you to the dark side is a pic of my door. The tape will be the size of the door (middle tube will go away) 
