what gain or loss to expect

At just 130 cylinder pressure, you really can't afford to lose any pressure.
Ima guessing you'd like a little more giddy-up, below 3500rpm?
496 in post#4 has a plan.
But really getting the pressure closer to 165 will really improve your giddy-up.This would require one of two things; 1) a smaller total chamber volume, or 2) an earlier closing intake valve.
The earlier closing valve will sacrifice top-end/midrange probably faster then it would build off-the-line torque. so that's not the first choice.
Reducing the total chamber volume ain't cheap.This is best achieved with bringing the pistons closer to the top of the decks. However, heads with smaller or closed chambers can do this too. But it seems to me your 68 heads are already at about 60/62cc. So we are back to pistons.
You can disguise the lack of giddy-up with a higher stall TC, or rear gears, but these mostly help with getting off-the-line.
Another option would be to lighten up the car, and is always my #1 go-to. They say loosing 10# is like gaining 1 hp at the big-end. So if your car can loose 200 pounds, that's 20 hp, and about a cam size in a teener.