I don't think my spindles have grease fittings, that sounds like a good idea! Are there any examples on here for where to drill and tap for the fittings?
I don't have great pictures, but I'll try to explain. The spindle units have a needle bearing in one and, and a roller ball bearing in the other. The grease fitting just needs to be somewhere in the middle of these, such that when you grease it, the grease will reach both bearings. When I tore my deck apart, it looked like noone had ever been into it before...and the seals and bearings were needing replaced. If you decide to go that route, it works even better to remove the inner seal from the bearing. For instance, the standard bearing will be a sealed bearing, meaning the it's a unit comprising seal-bearing-seal. Manually remove the seal from the side pointing toward the center of the spindle, so it is able to accept grease more readily from the zirk fitting. If you have a good local bearing shop, they should also be able to sell you a bearing with a seal in only one side, exactly for applications like these. I have part numbers somewhere if you need me to look them up.
The only other thing to consider about zirk placement, is the dust "cup" that might be there to cover the spindles from the underside. Mine were pretty well rusted out. I ended up using a hub bearing cap from a 7,000 lb trailer axle as a new dust cover, as it was the exact diameter I needed. That being said, I'm not sure the dust cups are really critical. If your dust covers are still in place, and you opt to keep them, put the grease fitting up high enough on the spindle that it won't interfere wiht the dust cover.
Here's some random pictures that may help. If they don't, then just disregard :)