1985 Chevy k10 gage cluster

[slipnslide]

1985 Chevy Scottsdale k10 .
Pulled 305 engine replaced with 1998 350vortec . I have replaced the wire harness with a original painless wire harness . I also replaced the gage cluster with a new one . The new gage cluster doesn’t have the clock but has rpm gage . The problem I’m having is the oil pressure gage when turn key goes to hi pressure and needle bounces when start truck it remains on high side . The oil pressure sending unit is new . Any suggestion why gauge is no reading right ? Thanks

 

[Turbo4whl]

Welcome to the forum. Some questions:

Is the replacement gauge cluster from the same year truck? Through the years the pin out on the rear connector changed.

Did you original gauge cluster have warning lights instead of needle gauges?