I use a Cortland 625 gr line with a 14 wt to good effect. For the line to backing connection I tie a 40-turn bimini in the backing, and then albright the loop to the fly line. I have not had a single failure at this knot to date.
I have been spooled (by a marlin) on an Abel 5 reel with 400 yds of backing, so now make sure I have 600 yds. I hope to never need all that, but it gives ma a bit more peace of mind.
sierra trading post frequently has heavy fly lines and heads at great prices. I keep an eye on their website and stock up occasionally.
I think heavy rods all load up, more or less, with 625 gr lines. I've used a Redington DFR 15 wt, Sage Rplxi 14, TFO TiCr 14 wt, TFO Bluewater heavy duty, and a couple others with the Cortland 625, and they work fine. The T&T 8'6" heavy rod was the only one I tried that wouldn't load (for me) with a 625 grain head.