Gears and stall make for huge seat of the pants improvements but how much you can stand with a 3speed tranny
meaning no OD and no lockup.
Actual power production is all about airflow. HP is all about burning fuel at the right air fuel ratio, getting more fuel in the cylinders is easy, getting more air in the cylinders to burn it is the hard part.
Edelbrock parts are often a genuine improvement over the OLD stuff like what you had stock BUT at the same time there is a LOT of room for improvement on the old stuff and Edelbrock does not take you as far as it could for the effort involved.
I mean the cam is single pattern, for a naturally aspirated gen 1 or even gen 2 smallblock that is a joke, these engines have always been comparably stronger on the intake side than exhaust which is why you will find most cams for them have more exhaust duration.
I understand using Edelbrock matching stuff to simplify emissions inspection, but at the same time nobody can see a cam during inspection.
Heads are a HUGE part of the HP equation since all the airflow in and out goes through them, heads would be a good place to look BUT I would probably stay away from Edelbrock, well made products that do not actually offer as much gain as they could. This is not to say go the other end of the spectrum and look at AFR either, they are FAR more marketing than anything else. I think the Vortec suggestion is decent, they are inexpensive and a lot of shops all over the country know how to make them work even better.