Very cool project!
I know this may be slightly off topic because you're discussing the frame here but.....
Have you done anything to the forks from the RC51? By "done anything" I'm referring to changing out the springs to compensate for the HUGE difference in weight between the RC and the Monster.
A while back it was becoming popular to swap out the stock non-adjustable forks on Monsters to some from a GSXR. After doing a few I found there was a one problem that kept coming up. The forks from the GXSR were way too stiff because of the weight difference.
Not only did the bike itself weigh more but adding all the extra weight for the additional fuel the bike carries, the weight for the coolant (air cooled Monster vs. liquid cooled GSXR), and the fact that the weight distribution is much farther forward on the GSXR creates a big problem with the stock spring rates.
In this instance consider that the RC51 weighs in at a staggering 427.7 Lbs dry compared to the 380 Lbs dry of the GSXR and you have even a bigger problem.
IMO, just something important to consider if you haven't already done so.