It'a nice little job, and I think you'll be pleasantly surprised just how much work you can get done on an anvil like that.
Hope you don't mind if I make a couple of suggestions though?
Here's one that I made a few years ago. This is actually the MkII version.
Cutlers anvil by
Mick, on Flickr
Cutlers anvil by
Mick, on Flickr
MkI was more like yours, but I couldn't live with the noise it made. Not the pleasant, bell-like ring of a proper anvil, it was a horrible clanging from the hollow section.
I ended up cutting it apart and welding a piece of 20mm thick plate to the top, cutting the hammer head in half at the eye, and putting a piece of old conveyor belting between it and the thick plate. This acted as an impact absorbing layer, stopped the metal-on-metal noise from the two parts and made the work absorb the energy from the hammer blows better by killing the rebound.
When you put the base on it make provision for bolting it solidly to the floor. As heavy as it will be, if you don't it will walk all over the place when you use it.
As has been said, if you do fill the hollow section don't use concrete. Sand, shot, or similar "loose" material will be far better. I didn't use anything in mine, and once it was bolted down didn't have a problem.