The 90TM heads are super weak, the worst of the bunch, they are pretty much guaranteed to crack. If an actual rebuild was done, chances are good the heads were replaced, but you can probably tell by checking them out, or if they are 93TMs and not 90TM, obviously they were replaced. Otherwise if they are the originals...I'd spring for 93TM heads. You can of course go for 95TMs if you can find them, but since you're looking for mileage rather than higher octane performance, new aftermarket 93TMs seem like the best bet, they will be stronger, and flow more just by design, without changing compression over stock.
You can't drain R12, it has to be evacuated from the system, preferably by a professional with the correct equipment. Does the A/C work though? Most of the stock systems usually develop leaks over time, and unless it's been taken care of or at least recharged, there might not be anything in there. You can press down on the pin in the service port on the low pressure side (above the drier cylinder on the passenger side) for a half a second and see if anything comes out. If not, hold it down more and see if it's completely empty. Be careful though, maybe wear gloves and use something to press the pin down. If nothing comes out you can probably safely disassemble the system, but still, be careful and assume there is pressure or at least oil/gunk in there that will squirt out at you.
If you're not gonna recharge with R12 again, you can spend the dough and get new hoses and stuff for R134a. I got all mine at Advance Auto Parts. You'll need new upper and lower hoses, a new drier, R134a switch, and orifice tube. They have an online coupon for 40% off, so you can get $100 of parts for $60. Pretty good deal when spending that much.