High pressure 235 replacing a low-pressure 216 into a 1950 Chevrolet truck:
"Do I need a new sending unit?"
As posted by Jerry, above: just attach the existing oil-pressure sender line to the new gauge.
"I do have a new (hi-press) Instrument Face to replace the "low" face."
Presumably, you are changing the oil pressure gauge "unit", not just the "face"?
https://www.classicparts.com/1949-53-Oil-Gauge-w_-White-Needle-0-60lbs/productinfo/24-702D/
