Just dropping in here... F1 engines are rules limited to 18k, a lot faster than 12k. They also have hydrolic valve springs. ...Had to double check whether they had cams or not!

One point: Must remove radiator/condensor to do a cam swap on an LSx. Atleast in an S-chassis. Saw a swap on TV where someone did a cam in something old. Step 1: remove bumper. LOL.

just one more... Try doing any sort of variable timing or lift with a pushrod. I think Dodge did it to the Viper and it was a huge pain in the dick.

Also, someone talk about area under the curve limitations on pushrod motors. I want to say that I've read you have to have really shallow ramp angles to keep the pushrods happy, thus requiring the huge lifts/durations and high flow rates to actually get a decent amount of air into the motor. Might be talking out my ass here, though.