Thumper.... need your help !

the block deck may be warped.

What are you using to clean the mating surface of the block and the cyl heads before you install? Can not be an oil based cleaner like carb cleaner, or degreaser [i'm sure you already know this]. I used brake parts cleaner on my Buick T Type, and i'll tell you what, I swear by it now Why?
I had an accident one night when I forgot to turn the wastegate down to 15 PSI after a track visit, and I had a patch put into my street and race chips for NO WOT fuel correction.. block learn multiplier locked onto a value of 128...[and I adjusted WOT enrichment with with fuel pressure]. Anyways, turned down this one road on the way home from work, and just floored it.. boost spooled fast, rears go bananas spinning, car lunges forward then all the sudden a loud pop..pop..pop.. and the car falls flat on its face like its shut off... guage is reading 23 PSI boost and it had about enough fueling for 15 PSI. ECM yanked more than 40* of timing because of the knock. Head gaskets were fine. Guy that has my T now has done some similar leaning out stuff... to the point of popping and the head gaskets are still sealing..

If the heads were cut by a reputable machine shop, I agree with the above comment about block deck surface.

My friend Kevin built a new motor for his notch, and like you, used 289 heads that were perfect when they were on the previous car [another buddy of ours]. His motor build started with a brand spanking new block. He told the machine shop to check the deck surface, and they said "yep everything checked out ok you should be fine". Get the shortblock home, gets everything together, and same thing as you... blows both headgaskets... Goes back to machine shop and has 289 heads checked they are fine. Asks the guy who built the engine himself.. "did you check deck surface like I asked". "Uhh No, it was a brand new block we never thought it was necessary"

Now the poor guy has parked the car and hasnt even uncovered it since... this was two years ago...I guess they are still in agreement to redo it for him long as the block deck surface is off

89.. if the heads were surfaced flat, and the head bolts ARE new, and torqued correct and in the right sequence, all the other stuff has been changed.... I had the same problem on the wifes t-bird... it was the rad!!! Yep, put in a new radiator... and the thing never over heats again!! and yes , I flushed it and kept coolant in it... just after the years... is started plugging, and then stoped up!! I could drive at 45 MPH all day, but when I reached 65... it over heated and pushed water!! SOOOO.... Give a GOOD look at the rad!! cool??

Just me..................................

Thumper