Hi Wayne,
I looked at my set of plans, and from what I can tell, it looks like the sweep-back angle is about 27 degrees. This is the angle that the extended gear leg would make with the vertical plane of the firewall.
As an item of interest, from the side-views in the engine mount drawing (DWG #33), the centerlines of the gear legs appear to be parallel to the angled footwell of the firewall. From DWG #22 for the F-401 firewall, I measured that angle to be around 26-27 degrees. Of course build variation in the firewall applies.
So, a couple of caveats here. YOUR airplane may be different. There could be some +/- variation on this angle . . . depending on the build of the firewall, the firewall alignment with the fuselage, the build of YOUR specific engine mount, and the potential use of any shimming between the engine mount and the firewall.
Also , I know you specifically asked about the angle with the top longeron. But I gave you the angle to the firewall instead because the top longeron is not straight & level for the full length of the fuselage. It makes a bend at the F-404 bulkhead (main spar carry-thru), so the longeron is pitched downward slightly between F-404 & firewall. If you built the airplane, you already know this, so please forgive me for telling you what you already know. However, if you bought it, you might not be aware of the bend in top longeron.
As for the front view (looking aft at the firewall, Drawing #33), it looks like the angle that the gear leg makes with the vertical is about 42 degrees. Again, YOUR plane may be slightly different. And this is the NO-WEIGHT-ON-WHEELS angle as taken from the plans I have, which are circa 1995. I do not know if the gear angles are different for the short-gear versions of the earlier RV-4s (mine has the tall gear). The sweep-back angle should be same regardless, but the outward angle may be slightly different in order to keep the wheel spacing the same on both short & long gear versions.
Best Regards!
