My last few cars were 240sx's and I've been drifting on and off since ~2003.
It's ALL in your suspension/tires. A stock NA6 Miata will drift just fine with a hair over 100hp with the right setup. The lack of IRS isn't really a concern either, as AE86's are live rear axle also and get sideways just fine with almost no horsepower (and, admittedly, much less weight).
Just get coilovers, IMO, and adjustable arms + adjustable panhard bar.
I don't know much about the tlok in a Mustang or how comparable it is to a true 2-way LSD from a drift car. I ran an ATS DeftForce in my last S14 and it was nasty to DD but got the job done.