Hey thanks, and seriosuly well done on your machop and Rapidash, they're awesome shinies, I'm gonna have to hunt them soon too.
Um as for hunting in the Pokemon Tower, well anytime that i do an RE hunt, i tend to train a certain Pokemon that can benefit from the EVs of the Pokemon in that area, or just level up any Pokemon sometimes. I know it takes a few seconds longer per encounter, but it feels like I'm doing something productive while hunting. So on the 5th floor of the Pokemon Tower there's a healing spot so i don't have to go to a PokeCentre in order to heal up my Pokemon. So long story short, it was actually quite a bit easier/quicker for me to hunt there. But you might run into a Cubone instead, if you don't want that, I'd hunt in D/P/Pt in the Old Chateau, because Gastly has 100% encounter rate there.