I've not done either of these kits BIAB but I've done Smashing Pumpkin several times and it's an awesome beer! I'm not quite sure why the SP calls for 4.5 gallons of water and the Irish Red calls for 5.5 on the kit instructions. The SP has 1.5 lbs more grain than the Irish Red so that will absorb some more water, but not much. On my system, I calculate grain absorption at .12 gal/lb so a 7.25 lb recipe should absorb just shy of 1 gallon of water.
If you boiled for an hour you can estimate roughly 1 gallon loss to evaporation (this depends on your setup). Both recipes say you should have roughly 4 gallons before starting your boil so I assume the 5.5 would be a better starting point. Maybe one of the more experienced BIAB guys can get you a better answer for that.
Typically when I was doing partial boils, once you chilled your wort down to yeast pitching temperature, I would add some water to the fermentor to get to your target volume before you pitch the yeast.