71 Shasta SC 16 and wanting to replace toilet

I was going to purchase the Bravura toilet by Thetford, did some measurements and noticed that it might overhang the factory "shelf" the toilet sits on.  Has anybody put one of these in a vintage trailer and had success?  I know the size is small, but the toilet overall length is 20 3/16, the old porcelen toilet that came out over all was 18 inches.  Not a big deal, but curious.  

Thanks
TJ