It doesn't really matter if the pistons are made for 6.0" rods, unless you plan on running them on 5.7" rods. I would certainly not recomend that, it would give you NO compression.
It will not be easy to get anything above 11:1 compression to run on pump gas, it is possible but it wont be easy.
I can't remeber the set you have, but I'm a fan or DART pro 1 alm heads, AFR, and brodix heads.
See, I dont know the size of dome in CC's that you have. I put this information into a compression ratio calculator: bore- 4.030", stroke- 3.75, head gasket bore- 4.100", quench area- .040", combustion chamber of the heads- 64cc, dome or dish of the piston in CC's- 6cc. and the result is 12.8:1. Now if you go with a 72cc combustion chamber head, it will put you at 11.5:1.
In order to run 11.5:1 on pump gas you will need a very well though out combination of parts to help lower the cylinder pressure to an acceptable standart for pump gas. Something that a novice or even a mildly knowlegable gear head would have a tough time doing. and it will require some of the best parts meaning $$$$, you will certainly have to have a decent set of alm heads and a cam with a lot of overlap and lobe seperation.
I strongly suggest seeing if you can return the pistons and see if you can get a credit for them on a new set of 18cc dished pistons.