I didn't know to look for a bypass.
Steve, NoMad said the pistons looked to be dished.
I've been searching for the H5611 stamped into the head of the piston and can't find anything. I can find H562* with * being a letter of the alphabet. Sealed Power was the only one listing an H piston. Mahle were all numeric.
Okay... You've been doing similar searching to what I have. (I'm looking at Rockauto.)
The iron head 3.1 appears to have been used in the Lumina APV through '95. It would have had flat tappet hydraulic lifters. Roller lifters, AFAIK, did not come in ANY iron head V6.
If you can't measure the stroke, you might try looking for numbers on the crank. That should absolutely narrow things down. (It would settle the 2.8 vs. 3.1 question.)
According to Rockauto, the iron head and aluminum head engines both had dished pistons. The aluminum head engines would have to have a larger/deeper dish, since the chambers were apparently smaller.
Some of the "piston" pictures would tend to back that up, if you want to believe the pictures.
I'm still thinking this is a 3.1 minivan engine.