I ran the 27x10.5 QTPs. You should be able to go 1.4's on those on a stock suspension; with minimal work, low to mid 1.3's. I ran around 16-17 psi in mine. That said, lighting up street radials is one thing, lighting these up at 75+ mph is a different story - they're basically slicks. How much power are you making? Is the road sandy or gravelly? Are your torque boxes, bushings and shocks/struts in good shape?
'84 LTD LX - 9.83 at 140.09. Whippled 365 SBF with a glide and 3.08 gears. Driven to and from the track 60 miles without even changing tire pressure.