I think one of the main points to consider is whether LB and DL are separate positions or lumped together (as many leagues seem to be doing these days). If they're separate, DLs will have value based on scarcity alone, but if not, you may want to raise the value of sacks a bit - otherwise, you might wind up with DLs being close to worthless.
With that offensive scoring, I'd probably go with something along the lines of:
Tackle, pass defense = 1
Assist = 0.5
Interception, forced fumble, fumble recovery, safety, blocked kick = 2
Sack = 2 (or more if no separate DL slot)
Defensive TD = 6
That'll leave you (just at a quick glance) with offensive players putting up somewhat higher totals than defenders; if you want them to be equal, you'd probably have to raise those numbers by 50% or so (run the numbers, though - I'm just estimating).
In any case, it's all basically a matter of taste, but these numbers probably aren't a bad baseline to start from.