Drilling and tapping the heads for acc. holes does not usually work. There is not enuff meat in the front of the heads and the outer surface is not flat. I've seen it tried many times and it usually always is at best a cobble job.
To address the hardened seat problem, check out the prices before you just drop the heads off. I've been in this business for over thirty years and by the time you pay for the seats and all the other things involved(guides, valvejob, mill, seals, etc) you can buy a 350 complete engine (or even a car) and have all the brackets you need plus a better engine for your purpose.