A very quick run with
1995 Magnum 5.9L specs on
Performance Trends Engine Analyser 3.2b computer program
using mods of

650 carb
dual plane intake manifold 1.72" effective diameter runner with 8 inch runner length flowing

shows 245 SAE hp at 4200 rpm
327 ft-lbs at 3200 rpm
with the stock factory streamlined Magnum manifolds and stock exhaust

286 SAE hp at 4400 rpm
364 ft-lbs at 4000 rpm
with a change to
forty-two inch long 1.625" internal dia primary
4 into 1 tube headers 6 inch long collector
and a 550 cfm rated aftermarket exhaust that gives less than 2 psi backpressure at 5200 rpm WOT

for 'standard dyno room' horsepower numbers
add 2% or more to the above numbers

Kevin at Performance Trends only claims that this engine simulation program comes within 7% of actual engine tests,
but that dyno to dyno variation is about that (or more)