A rushing FB will want as much Power Through as possible, and for an inside runner I believe Dive For Yardage is important as well (Based on two almost identical HBs on my old team, where the only real difference was 4vs1 in Dive 4 yds. The one with 1 in the skill even had higher speed, strength, agility and vision. Resulted in a .5 ypc difference over 180+ carries each, in favour of the back who was worse in every aspect except that SA)
However for me as a blocking fullback (In fact I have three. Although the first one is balanced but focused on blocking) I really don't know how effective the SAs are. I have roughly 3-3-3-3-2 for my lvl 27 and 22 FBs, and I've had so for a long time as I don't know whether it's worth it to get more. I mean most of the time he's blocking LBs, Safeties or double teaming D-linemen. For that you don't need a great deal of blocking. The 65+ strength and 45+ blocking is enough for that. Pancake would, obviously, increase the # of pancakes, but looking at replays it doesn't seem like that would do a great deal to help the HB.
Strong Arm and Lead Block, based on their description, essentially work the same way as more strength would: helping you push defenders backwards. But if you're already doing that, why get them? Lastly, Spot Blitz. I'd put points in there if I knew whether it helped SPOTTING the Blitz in addition to what the skill says for sure, that it's a +blocking bonus.
If anyone has experience with these skills at a high level, I'd really appreciate some advice. As I'm at the stage where my physical attributes are enough for blocking (Strength and blocking can always be increased of course, but I'm past the soft cap), so it's a choice between going for blocking SAs or focusing more on running. And I'm only going down the rushing route if the blocking SAs really are useless, as my FB doesn't carry the ball.