I agree, you've done all the easy & obvious stuff, what remains is far more labor intensive and expensive for much lower return in power per dollar spent. More cam will certainly add power, but low speed flexibility and "manners" in traffic will suffer. A "looser" converter will help there but $$.
Depending on your budget, fabrication skill, favors you can call in, etc. forced induction is the logical choice.
The Roots (4-71 Eaton etc.) are traditional, the Eaton is cheaper (used) but they make a large package.
A centrifugal (ProCharger, Torqstorm) is easier to mount alongside, drives off another belt.
A turbo is a lot of plumbing including the oil system, but the turbo itself can be cheap ($500).
