The vortec heads are the best production SBC head.  They flow 238 CFM @ 28" vacuum and .450 valve lift.  The '88 5.7 heads flow nowhere near what the vortec heads do.  The vortec intake has an eight vertical bolt bolt-pattern compared to the twelve angled bolts of the earlier heads.  You can redrill the intake bolt-pattern, but the ports don't line up as nice as the correct manifold.  You will also need the correct rockers and push rods.  You might run into issues if you ever have to go to a smog referee.  Unless you are going to an aluminum aftermarket head, IMHO, your performance will drop compared to the vortecs.

The vortec heads are 1.94 intake and 1.50 exhaust.  They are very well shaped valves without the characteristic "ski jump" that most production valves have.  I scrapped a few brand new sets learning how to port them and I am pretty sure I still have all the new out of them.  They work really good with 2.02 and 1.60s, the low lift flow numbers pick up substantially.

Look around, I bet you can find some good used pull-offs cheap.