Stock Vortec L31 springs/retainers are lift limited to 0.450", not 0.475". At lifts over 0.450", the bottom of the retainer crashes into the valve seal. Also, the springs are too weak to consider any cam change over bone stock.
The valve guide boss is thick on these heads, preventing use of a performance spring with the stock 1.250" O.D.
There are a couple of fixes. The one I like best is to cut the valve guide down to 0.530" so you can use a good stock-diameter spring with damper and use PC valve seals. Not that there is anything wrong with the stock seals. They work great. It's just that with the thick guide, there isn't enough room for a good spring and damper with the stock spring diameter.
The other fix is to leave the guide and seal alone and cut the spring pockets for a larger diameter valve spring with a higher installed height so that the retainer won't interfere with the stock valve seal.
For cutting the guides, use a CompCams cutter (SummitRacing part # CCA-4726 $39.25) and a CompCams arbor (SummitRacing part # CCA-4732 $12.00)
Finish off with a set of Z28 springs and retainers and 0.530" PC seals from Scoggin-Dickey Chevrolet (part # KITSD1004 $66.98 for the springs/retainers/keepers) and (part # SD1711 $19.20 for 0.530" seals). These springs install at 1.735" with 100 lbs on the seat. Open pressure at 1.210" is 285 lbs. They coil bind at 1.160" and the max theoretical valve lift to be used is 0.525".
The choice of cam will be determined by the static compression ratio of the motor and that will be determined by the stack of parts used, the block deck height, piston deck height, gasket thickness and chamber volume. Please, please do not just reach out there into thin air and choose a cam without going through the numbers and choosing it scientifically. If you don't know how, PLEASE ASK. I or any of the others on this board will be pleased to help you.
Unless you cut the rocker stud bosses and install screw-in studs with guide plates, you will be forced to use rail rockers like those that come stock on the L31 heads. There are aftermarket rail rockers in 1.5 or 1.6 ratios. Scorpion makes a nice roller rail rocker for a reasonable price. If you use a 1.6 rocker, pay attention to the lift on the cam and don't exceed the 0.525" max for the valve springs. Also pay attention to the pushrod clearance where it comes through the head.
An RPM high-rise intake manifold will show 30 to 60 hp improvement over a stock type Performer low-rise intake manifold, but you may not have room for it on your Vette.