I see! 'Here' a planter like that is going to run more than what a new fert tub from Shoup would cost, so it was confusing to me.....
I've never done it, but cutting down old planters into feed plot planters for the 10 acre homesteaders and deer hunters is a big business, hence the price of old planters... Again 'here' talk of scrapping good planter units would be crazy talk. ;) I believe you would shorten the frame to accommodate 2 rows, and then move a pair of rows to fit on the planter, with a fert tub, and work out the drive. I believe it would be hard to move the hitch and it would be hard to keep the 2 inner row units. You would want to lop off 2 rows after taking everything off the main bar, and then place either the left or right pair back on the planter.
The 2 extra row units would be converted into a single or double 3pt planter as well, no markers, just need to savage a drive wheel assembly from somewhere.
They ask $1400 for a setup like that with fert tub around here, so you would get over $5000 for your 'scrap planters' around here with some time and a spray can of paint invested.
Enough worthless babble.
Have you used a front mounted cultivator much? I ran some for a couple decades. Maybe you are on very dry land, but here, again, you would not ever want 1000 lbs more weight on one. Trying to get through the wet spots is an adventure as it is, without any extra weight!
I like your ingenuity, but think this will be a long work in progress until it becomes a happy solution for you.... For a few acres like you have a smaller setup would work, and when you get bigger this will become too cumbersome, I fear.
You really need a ground drive for it, off the rear axles or a drive wheel, so you can always match fert flow to ground speed. Hyd or pto or electric drives will add more complexity as you can't easily keep your fert rate matched to your ground speed. I remember cultivating, it takes a lot of more throttle from time to time.....