You will see that most improvements that increase the amount of air an engine can draw in will increase the horsepower, but reduce the torque at the lower RPM levels. However the maximum torque is increased, being produced at a higher RPM. This is because improvements to airflow slow the incoming air velocity at slower speeds, but increase the air volume at higher speeds.
You will see that the CompCams XE268H cam improved every level of the power curve.
This is partly due to the improve design over factory of these Xtreme Energy cam.
Using a shorter duration XE cam will most likely provide a greater increase in torque at the lower speeds. However in the "real world", engines don't always respond as expected.
For example the 1-3/4" headers should have reduced the low speed torque, yet they improved it. Using these bigger headers on a stock motor would surely reduce the torque.
Additional, two identical motors will most likely produced differing power output numbers, never the less they will be close. This is because there are so many subtle variances in a motor, from port shape and size to ring sealing.
You will also see that the now famous GM Vortec heads only increased torque above 5000rpm,
but this increase becomes substantial. For an engine turning at 6000rpm (the numbers in blue are at 5800rpm) there was a 33ft/lbs increase and 45ft/lbs with the ported heads. The horsepower was increased 38bhp and 51bhp respectively.
These are significant numbers, but the increased cost of using these heads, typically requiring a valve spring upgrade and a special intake manifold, as is the case with most performance parts is only worth it if you plan to "rev" the motor up.
A slower running truck motor for example will not benefit from these heads.
Never the less a Vortec motor also uses a hydraulic roller cam, which will also improve overall performance, and was the staple for GM pickup trucks through the nineties.
If you are replacing a later model vehicle motor the Vortec is a good choice.
If it is an older vehicle, or you don't plan to rev it beyond 5000rpm, or you don't want to spend any more money than necessary, then the early GM Goodwrench motor can serve you well.
One last word on Vortec heads. They are cast very thin and are fragile if overheated and will crack serious, well beyond repair. However these heads will out flow Dart S/S, World S/R, (to.550 lift) GM LT1, Corvette L98, Dart Iron Eagle 180cc (however their better ex. flow will likely result in greater power production).
Pro Topline makes a much stronger Vortec head, and Edelbrock makes two sizes of aluminum Vortec heads.
Pro Topline's Torker head will out flow both their own and Gm's Vortec head.
A word on 383 conversions. With no other changes except the longer stroke, theoretically a 383 will make 30ft/lbs more than a 350 below 3000rpm, but begin to loose this advantage as the RPM climbs.
This is due to the increased friction of dragging the rings further up and down the cylinders.
We have found that in the "real world” the horsepower lost to fiction is not quite as serve.
However because the 383 is larger, using a slightly longer duration cam will produce a similar idle quality and vacuum as a smaller cam in the 350 and improve the upper RPM power.
Overall a 383 will accelerate more quickly in a street driven application, but a 350 will begin to catch up eventually. It is unlikely to make up the difference within a 1/4 mile.
Finally a roller camshaft will improve the overall power output, although it may no be as much as illustrated here, See CompCams Testing.
We have found that if available for an application using 1/6" rings in place of 5/32" improves power due to less fiction lost to ring drag. Lighter weight pistons and rods also promote more rapid acceleration, be the results in a street curser do not justified spending too much money on exotic parts.
I hope this information helps you determine where and how to spend your money.