Depending on what you're looking to do and how much you're willing to spend, here are some cheap options from Minneapolis Craigslist for you to consider. I dunno if your engine is the original engine to the car or if it's swapped from something else, but some of these may be an upgrade for you.....
I'm sure these wouldn't gain you much:
And here's some with a little higher price tag, but most likely would bump your compression:
Ford 302 Heads
These have been under my workbench for 25 years. Supposedly fairly fresh rebuild by Gopher when I got them. E-mail with phone number.
minneapolis.craigslist.org
Ford 302 heads
New lower price! A pair of complete Ford 302 heads. Unsure of the year.
minneapolis.craigslist.org
I'm sure these wouldn't gain you much:
Ford 302 heads
D80e heads off a running ford 302. Needs to be cleaned $40
minneapolis.craigslist.org
And here's some with a little higher price tag, but most likely would bump your compression:
Cylinder Heads and Covers for 289_1965-1966 Mustang
Cylinder Heads for a 289 non-HIPO 1965-1966 Mustang. May need to be rebuilt. Head covers included. Were sandblasted and repainted Ford blue to match the heads - there is tape over the intake hose...
minneapolis.craigslist.org