NV3500 behind a 340?

try this
New Venture Gear 3500 transmission - Wikipedia

The New Venture Gear NV3500 at a glance

rated at 300 or 340 ftlb; depending on which unit you score.. A 318 can make that.
It's hard to imagine a 360/5.9 making that little torque..... at WOT. So you should be OK with a tiny tiny 2 bbl on your 5.9, and/or skinny tires, and/or just don't floor it. Buuuut that's not why you installed a 5.9 is it? lol.