I use Performance Trends Engine Analyzer "Pro". It's a decent simulator. We use it primarily to plot power curves and anticipate results from changes. As for the "peak" numbers, I've seen it anywhere from dead-on to 15% "high" based on engines that went to real dynos. So take the numbers with a "grain of salt". I'd be willing to say they're within 10%, reliably.
When I saw the XE256H cam listed, I realized the "guessing" going on was based on smaller engines or larger cams. I've used this cam in both small blocks and Pontiacs extensively. The 400 Chevy "likes" cams more like the Pontiac than the 350, IMO. This cam will have pretty much a "dead smooth" idle in a 400. Stock converter is fine.
I "assumed" Performer, as he mentions Edelbrock, and RPM isn't appropriate for this cam. The 650 carb IS a bit small, I agree. But with this cam, it works to the "curve" the 400 has. I also "assumed" 1 5/8" headers, "open".
Following are the numbers generated, using the real data for all the parts, 93 octane gasoline, 165 deg. test temp, (note the program assumes "perfect" air and timing).
Peak torque: 482 lb. ft. @ 3,500 RPM. Over 400 @ 2,000. Flat curve. Still made 440 @ 4,500.
Peak HP: 377 @ 4,500 RPM. 322 @ 3,500, 358 @ 5,000. Nose-dives after 5,000.
Calculating to 9.56:1 static compression and dynamic compression ratio of 7.60:1, and estimated vacuum at idle, 17.7". It doesn't "like" an aggressive spark curve until 4,000 (shows "spark knock" under that).
Based on all this, I'd say the torque numbers are a bit high, but HP numbers are a bit low. Having used that cam in several 400 Pontiacs with stock heads, the curves fall "in line" with an engine of this displacement. The Pontiac will make a little more torque and a little less HP, but the ranges are the same. For those unaware, the Pontiac and Chevy 400s are close enough in bore/stroke and volumetric efficiency at these speeds, the data is "good" for either. The longer rod and taller deck makes the Pontiac more of a "torque" engine, and the shorter rod and good heads make the Chevy more of a "HP" engine.
I would say this combination would be dynamite in a heavier car for "sporting use", not racing or serious performance. It might get pretty decent gas mileage, too. Even though I'm well known as a Pontiac "bigot", I'm not stupid about Chevys... (:- The 400 has SERIOUS potential when properly built.
If you want to "move the power up" just a bit, and still enjoy GOOD low-end, use XE262H instead. The 750 carb with that cam would really wake it up. 1 3/4" headers would be called for here.
Sounds like a nice street engine, either way!