The only rototillers that can go to 8" or deeper are the largest and heaviest ones made, which weigh around a ton, are 8-10' wide, and usually cost in excess of $10,000 new. They are very uncommon to find used. A 6' King Kutter or similar rototillers that only cost a few grand new only till about 4-5" deep at most. A Deere 5400 or 5420 is also going to be about 6 1/2' wide with its tires set in at their narrowest setting and a 6' tiller won't cover the tracks. It's not something somebody can't work around but one really should be looking at least at a 7' unit behind that size of tractor.
I'd recommend a plow with 16" or wider bottoms if you want to turn over soil 8" deep as your plowing depth is half of your bottom width. Plow up the soil, let it sit for at least a month, and then disc it or run a rototiller over it.
You don't need a creeper to run a rototiller, just keep the speed in that 1-1 1/2 MPH range and they work well. Most newer tractors can run at least that slowly and still turn full PTO RPM with a regular transmission. The 5400 and 5420 run between 0.7 and 1.4 MPH at full PTO RPM in first gear without a creeper depending on the tire size, transmission, and whether or not the 5420 has a 540E PTO (the 5x00 series don't have EPTO.) 5 PTO HP/foot is a pretty good estimate of how much power a typical rototiller takes. It doesn't even take 15 PTO HP/foot of working width to break new ground with most plows, and plowing is a lot harder than running a rototiller.
