Everytime I think about the L31 heads, I get a little less excited about them from a total cost standpoint. The only thing they have going for them from a hotrodder's standpoint is that they flow well.
1. Thin wall casting, prone to cracking.
2. Pressed-in rocker studs, prone to pulling out with stronger springs. I seem to remember that the stock springs are about 260 lbs over the nose of the cam. Either need to be pinned or the heads need to be milled, drilled and tapped for screw-in studs. Pushrod holes will need some work too. Guide plates will eliminate the rail rockers.
3. Stock guides are too fat. You can't install outer springs with a damper spring without cutting them to a smaller O.D.
4. Stock guides are too tall. You can't run any more than about 0.420" valve lift with them if you use the stock retainers and valves due to retainer/seal interference.
The only way I would use these heads unaltered would be on a mild street motor. I'd keep everything stock on the heads, including new, stock springs, new seals, rail rockers and use a cam with no more than 0.420" valve lift. Some heads will suffer from the seals not being pressed all the way down on the guide, so use a socket and small hammer to seat all of them down snugly.
Here's an example of a combination that might work....
Cheapo to build, cheap gas 350 street motor....
These pistons...
http://www.summitracing.com/parts/UEM-KB142-030/
This cam and lifter kit...
http://www.summitracing.com/parts/HRS-CL112011-11/
L31 heads from a boneyard, casting # 12558062 or 10239906. Magnflux 'em for cracks. Fit new STOCK springs and seals. Make certain seals are fully seated on the guide boss by tapping them with a socket and small hammer. The stock springs will work well with the mild cam and will not pull the rocker studs out of the heads. Doesn't matter whether the heads have hard inserts in the exhaust or induction hardened seats, either way will work. Same casting, both styles flow the same. Maybe freshen up the valve job and otherwise leave 'em alone. Use the rail rockers that came on the heads stock.
Align-hone the main bearing bores in the block and cut the block decks for 9.015" block deck height, referenced off the main bearing bore so that you know the block deck height is equal on all four corners. This will insure that the heads sit level and flat on the deck and that the intake manifold will sit square to the heads with no gaps to cause an internal or external vacuum leak. Use GM head gasket #10105117. Static compression ratio will be 9.1:1 and squish will be 0.042". Use Edelbrock #7116 Performer RPM intake manifold. 600/650 carb is all you'll need.
Makes 365 hp @ 5000 rpm's with a double torque peak of 420 ft/lbs @3500 and 4000. Should make this power on crummy cat-pea pump gas without detonation and make a real nice daily driver motor.
Cam begins making power at 1200, so it's your call whether or not you need a looser converter. If I did anything at all with this combo, it would be a mild 2000 stall unit just so I didn't have to contend with the car creeping at stoplights and also to get up into the 1500 rpm low range of the intake manifold when the light turns green.