correct. you've gotta leave a couple wraps around the drum, with cable on cable, so it gains friction. otherwise the end will almost certainly come off.
DD, you're crackin' me up. in my browsing, you're always fabbin' something up for that tractor. i guess when you've got that much steel/tooling around, you're obliged to use it.
so, i've been doing similar projects, these last few months. i'm running a winch operated dozer/snow plow on my '03 dodge cummins. my setup is an old dearborn 6' blade, which i converted to hang off of a pair of eyes, 2' apart, and welded onto my front receiver. i also run my winch/snatch block off of an upright out of the receiver.
now, i'm probably 400#, or so, but i did start off with a 2000# winch that grenaded when i caught a patch of frozen dirt. it also flung both of my headlights off of the upright, and onto my hood, when it snapped back.
round two. 10k# winch. that one tore my upright clean off of the stinger, for lack of gusseting. small gusset added. no trouble since then. well, aside from the spool in solenoid sticking, and destroying all but the winch, itself, as it wrenched part of the fairlead plate off, the snatch exploded, the guard bars ripped off the drum, and a chunk of housing taken with the 2nd guard bar. se la vi. then i relocated the solenoid box under the hood, so it wouldn't take on water anymore!! read: limiter would be cool.
now, on to your scenario.
your 1500# winch will only hold up a few times at maximum load. at 750# it will hold up for a week's work. at around 250# it will hold up for the summer, and maybe two.
looking at your geometry, you're placing the lift point well short of the load. meaning, most of your weight is beyond it. at that location, 400# of dirt will actually put something like 650# of pressure on that point. if you pull from anything less than perpendicular to the lifting arm (angle changes when raising), then you can easily triple that load. ideally, the lift direction should be from somewhere around a foot above the nose of the tractor.
doubling your line must remain at acute angles, or you add incredible strain over around 60*.
with my current setup, i extended my plow out away from the truck, so i wouldn't "bronco" up over it when i hit an obstacle or heavy load. this made my doubled line angle fairly large. too large. but that's where having an oversized winch comes in handy.
lots of failure points on an underrated winch: motor, gears, solenoid, cable, cable ends. all must hold up during load.
sorry to ramble, but i've been through this all very recently.