A 4L80E is a good transmission. As you no doubt know GM put it behind the 454 and with the 5 pinion overdrive, behind the 496. You will not stress it with a 292. The problem with this setup is not enough gears -- only 4 and 1st is higher than the 4L60 making the gears a bit closer together in the interest of trans longevity. Even so, the engine you build will need a broad, flat torque curve like the 454 has or the stock 292 has.
I am on a road trip at the moment with an AD pickup loaded with a camper and grossing about 9,000 lbs. The 261 does just fine even though it's a bit peaky with ~254 degree cam, about 220 at 0.050 lift. The only reason this works on the road is that I'm running what amounts to a 10 speed plus 3 ranges of compound low. There is about 400 RPM between gears. From Bakersfield to Barstow I only had to drop to 3rd and over because I got caught behind a slow motor home on the Tahachapi grade. The rest of the time I just work the top three gears. When I'm gearing down for a grade the fire pretty much goes out below 2500 RPM so shift early. Torque peak seems to be about 2900 and the horsepower comes on above that. I turn it up to about 3300 to make the next gear on the way up if I'm pressed by traffic.
l hope this road tale tells you a little about what to expect when pulling hard. If you're limited to a straight 4-speed you need a very broad, flat power band and the "RV cam" I use won't do it for you. You need good intake velocity at 1800 and power up to 2800 RPM. You're going to see about 1,000 RPM between gears with the 4L80. I have one behind a 454 that I tow a 15,000 gross rig. That's a 98 Vortec 7.4 and I keep wishing I had at least a 6L80 trans even with that big engine.
