Most of the gear guides will be focusing on raid boss tanking. Over at maintankadin there is one post re: Pre-raid tanking that does note specific items as "good for raid tanking" and "good for 5-man tanking"... the 5-man stuff is closer to what you'd be looking for. In addition to stacking block, stacking +spelldmg is a great idea for what you've got in mind. Cons, BoSanc, Ret. Aura, Holy Shield, etc... all spell damage.

You aren't going to need 490, like you said it assumes a level 73 mob. Defense decreases the chance of crits (regardless of level) by .04% for each point that your def is higher than the mobs weapon skill.
(theorycraft here).

So, a level 40 mob has 200 weapon skill. At 70, your base def is 350, that's+150 on them, and 150 * .04% = 6%. A mob at your level has a 5% chance of critting you (+.2% for each level above you). Now I have no idea if a mob lower than you has less than a 5% chance, but even assuming they don't, 6 is a bigger number than 5, so you're good with just base def. (Aside: Resillence also figures into this equation, but it's not relevant for our purposes due to the level discrepancy).