It would involve swapping more than just the heads. The 12V has a drive for the distributor, while the 24V used coil packs. This would also probably mean changing the whole engine management system. Another difference is that the 12V has a mount on the right head for the alternator that the 24V doesn't. Actually, all the accessories are located differently on the 24V engine. Physically, they will bolt on, but you will need to do a lot more research on what else has to be changed. It could be done, but I'm not sure it's worth it.

Lee
'94 LS, 3.0 V6, Auto, completely stock