I believe the port shapes are a little different. The early 5.7 stuff has a small notch for the injectors and is a little more rectangular as I recall. I think the 6.1 stuff is pretty square and was more or less the design they kept on all the next stuff since. I almost want to say the 6.1 port was even a tad smaller, but that might not be right. O-ring vs flat gasket is probably a non-issue. My ModMan intake never had o-ring grooves, though I wish it did. It just used some phenolic style gaskets to seal things up. It would be nice if they could be adapted as that is one thing that keeps me from using a newer engine if I ever had to replace mine.
I would love it if someone had every version of head that was made and lined them all up in a row for a picture to compare intake and exhaust ports. I almost thought about posting and asking that recently when I started searching myself. Anyone have an early 5.7, Eagle 5.7, 6.1, 6.4, and 6.2 (Hellcat) head all sitting in their shop? I think the Hellcat head is just a 6.4 head, so might just be the 4 to compare (early 5.7, late 5.7, 6.1, and 6.4).