Thematic Backstory for Everyone:
Hollywood Blvd when you enter is in the 1920s moving into the 1930s. When you turn down Sunset Blvd, you were supposed to be essentially traveling through time from the 1930s into the 1940s. Once you reach Sunset Ranch, this is evident by the refrences to the war. When you reach Tower, one could argue a few things, you are stuck in 1939, you are sometime in the late 1950s or early 60s when Twilight Zone was on, or you are in present day.
RnRC was originally supposed to be built near 50s Prime Time in its early concept with Elvis blasting through the ride from what I've read, but it quickly evolved to Sunset. Gforce records started out in the jazz and big band era. Following the incident at the hotel, business faded and soon a new type of music was entering the scene. In the 50s, the big guitar was built to attract the new business of rock n roll. Unfortunately, when you enter the courtyard you are now fast forwarded in time to the present day (1999). The transition is about as effective as the last act transition of Carousel of Progress. Its a stretch, but not ideal.
Now, to make Marvel work on Sunset, I can only see Captain America because he fits the time period and that cant be done as an overlay.
Gardians of the Galaxy has no place on Sunset Blvd. Period. You can't simply tie the music from the film into Gforce records without completely turning the attraction upside-down. How do you transition the exterior, the preshow, the alley? Tower is even worse and more heartbreaking to think about.
You can successfully transition from Sunset to Marvel but not with Guardians of the Galaxy, not as an attraction overlay, and not during the time when the park can't afford to lose a headliner attraction for even a month all for the namesake of synergy.