Another route you could go is to get some L89 corvette aluminum heads from a late 1980s TPI Corvette. These heads are the first vortecs and have D shaped exhaust ports, 58cc combustion chambers, 1.94/1.5 valves, angle plugs, center bolt valve covers and pre-86 intake bolt pattern. I believe Chevy uses a modified version of these heads on the ZZ4. These heads will require at a minimum that you do the following: i.) get exhaust manifolds (such as the LT1 exhaust manifolds 1992-1996) or headers that match up to D ports and angle plug heads; ii.) get new valve covers for center bolts; iii.) drill/mill/grind the four ("4") center bolts on your intake manifold and fabricate/buy washers (should take about an hour) so that the intake bolts can get a good "crush" on the gasket. I have found "hillside washers" to be more time effective than fabbing up such washers. See fastenal catalog for hillside washers -
http://www.fastenal.com/catalog_pages/2005/0159.pdf
Please note, what you are in essence doing is putting a post86 intake on a pre86 bolt pattern (even though the L89 heads are typically from 86, 87, 88, 89 corvettes - yes I know its a little confusing). This is the opposite of what most people are doing (namely most people are putting a pre86 intake on a post86 bolt pattern). Holley makes a kit for such a situation. See
http://www.holley.com/90748.asp I am unsure whether this holley kit will fix the angle problem (as it is intended for the opposite configuration) but it seems awful pricey at $85. Finally, depending on what cam and rocker arm ratio you are running, you may need to inspect/clearance the valve as the bore on the 305 is substantially smaller than the 350. I have heard that as long as stated lift does not exceed 450 with 1.5 rockers than you should be OK. Additionally, I have heard that you can go to 500 lift with a little checking, clearance work and maybe new valve springs.
There are a set of these heads rebuilt for sale on ebay for $600. See
http://www.ebay.com/itm/tpi-corvett...ies&hash=item2c643846c3&vxp=mtr#ht_500wt_1202 the auction is entitled "tpi corvette 350 aluminum heads 10088113 sbc chevy zz4 z28 gta trans am"
These heads will give you better flow, more power, better mpg and more low end torque.
