I have a set of GM iron heads for my 350ci camaro which came from a drag race corvette.They have manley 3 angle valves,miloden studs,comp cams springs etc and appear to have been extensively re-worked.The casting numbers on them read (as you look at the head from left to right from above)
3978487 GM
L 6 71 34

and as you look underneath there are 2 numbers 487 and 493.
Any help or links to any sites who deal specifically with heads would be much appreciated.

I think you mean these..

3973487....71-72...350..........used on 71-72 350 LT1, 75cc chamber

someone else will have to tell ya on there potential..

Good is a reletive term. They're pretty decent for stock heads, and the "reworking" can't hurt. So I'd say better than most, but still won't touch a set of good aftermarket heads.

They'll lower compression.

It's OK because the hypereutectic pistons I have will take the compression to 9.5:1 with these heads.Incidentally the (burnt out) car these heads came from had a load of cash spent on it,such as gear vendors under/overdrive unit,TH400 with loads spent on internals,aftermarket injection unit,roller cam,forged crank and pistons,B&M blower and loads more I can't remember.The question is,if a guy has spent that much money on the rest why did he use these heads when he could have used anything?(he wasn't short of a few quid to buy a gear vendors unit).

What's a quid?

The lb, Its Bloke Money, Somewhere around 2 american $$

I imagine its a little harder to find f-body parts in the ol UK huh....I was stationed at RAF chicksands for 2 years.....I love your country, I have a 19yo daughter who was born there....