bore it if it needs it...it probably needs it. Boring an engine is more for reconditioning than making power.
as far as the heads go, 305 heads are usually 58cc combustion chambers. larger chambers will drop compression. World Products makes a S/R305 head that has the small combustion chambers for a 305, but they have larger valves & the ports move more air.
Another good head is the Vortec head, but remember, the vortec head uses a differant intake, valve covers & self aligning rockers.
Bigger isnt better, & this is espesially true with small bore engines like a 305. stay conservative on the camshaft & carburation, as well as compression.
The last 305 I built , I used KB143 pistons,L-98 Corvette aluminum heads, a little Comp cams XE256 hyd. cam, a Performer intake, 570 Holley, 1 5/8" headers. it ran 13.70s in my 86 Caballaro with 3.08 gears.
You would be better off dollar for dollar putting the 305 off to the side & building a larger engine...350 for example. However, a 305 can make decent #s as long as you remember your building a 305, not a 350, so stay conservative. John