I think the biggest glaring difference between the 3-4 of old and the 3-4 of now is at the right defensive end spot. Richard Seymour is the old ideal and granted, he was a special player who could not only destroy a blocker but get pressure on the quarterback.
Seymour was 6’6", 310 pounds. Now we’ll see someone a little more athletic there whether it’s Chandler Jones (6’5", 265) or we even had Chris Jones (6’1", 300) there a bit this preseason.
These guys are meant more for pass rush than necessarily being able to “build the wall” as a two-gapper. This is a result of the evolution of the game. The old 3-4 was not a great pass rushing package because you’re essentially just asking three of your front defenders not to get upfield, but to hold their ground.
Now you must have guys who can disrupt the quarterback from your base package, thus a shift in size/style at right defensive end.
Theoretically could they put together a rough version of an old-school two-gap 3-4 with Kelly-Siliga-Wilfork upfront? Yeah, but I don’t think we’ll see that unless a team decides running the ball 50 times is the way to go.