The difference between the 300 and the 350 are engine and interior finish . The 350's torque is a noticeable "seat of the pants" improvement over the 300. Dead stop take off acceleration is no contest. The 350 is quieter at idle and at speed making interior noise less. Overall drive-ability improved over the 300.

The 300 will be a little faster from 50-80 mph due to greater HP. This may be an advantage of the 300 on the open road. The 350 handles any up hill grade at speed with ease.
The 300 might be better at passing on a 2 lane road .

The 350 interior is an upgrade to the 300.

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1990 350 SDL
Currently: 180,000 miles
Factory rebuilt bottom end rods, bearings and head gasket @ 75,000 in 1997

Thanks for the explanation.