That Schmidt Motor Works engine designer program is pretty buggy,
even if you know the harder to find parameters,
and can work around its asking for 'throat velocities' instead of the more common flow bench cfms.

On the very first run using the factory spec's of a 1995 model year Magnum 5.9V8 it gave some optimum intake manifold lengths that were 'in the ballpark' at 18.4 inches for the 2nd harmonic... but after that first run it kept reading NaN

As others have said above,
the estimated HP, torque and maximum RPM limited by piston speed
were very high compared to measured SAE values, even making allowance for AFR differences and the different camshaft specs it declares 'optimum'

Compared to the PerformanceTrends Engine Analyzer 3.2b I would consider it harder to use and less accurate.

The table of harmonics for the intake runners lengths is interesting... if it would calculate consistently and not keep giving NaN, whatever that means.

The primary pipe lengths and inner diameters of the headers seem to be in the ballpark.
The estimated EGT would be interesting to compare to the real world, but at first glance appear low.

you can't beat a price of free
but that program has major faults