Leroy you must have pool table flat ground. If you have to plant through a low spot, with a planter with press wheel gauging, as you crest the low spot the corn gets planted deeper then as you get to the bottom the corn is planted shallower.
Also a shoe type planter is a PIA to get to work is some conditions. Such as heavy grass sod. Even plowed there are still chunks of sod that will drag in front of the runner and then you have your corn planted into a ditch.
As for seed box height. I just went and measured a JD 7000 set on the ground. Plus I have a 494 that I used to plant sweet corn with. So I measures it on the ground. The 7000 is taller. A whole three inches to the top of the seed corn boxes!!!!!!
As for pulling harder. The guys where just using bigger tractors as they had them . I pulled a JD 7000 6 x 30 planter for years with a JD 2020. I never had any problems pulling it even full of fertilizer.
So I guess you just don't like to have a good stand and more uniform emergence. You try to get 200 corn with an old 494/694 and you will have a hard time getting a good enough job done unless you have perfect weather and soil conditions.