<VV> Oil Pressure and late Speedo cable install

[Paul]

There is probably no question that hasn't already been answered, so 
please bear with me.
1) I purchased a new speedometer cable for my 65. How does one get the 
old firewall grommet off the old cable and on to the new cable? There is 
no way that I can see to be able to stretch  the grommet, with its' 
metal insert over the "nut" at the speedo end.
2) Oil pressure.. I have a new engine and a new electric (Auto Meter) 
oil pressure gauge. It reads a max of about 25-27 psi. An older Faria 
electric gauge reads the same.( Each was used with its' own sender) 
However, an old mechanical Stewart Warner gauge reads 40-43psi. a 
mechanical gauge at the local speed shop reads 44 psi, as well. I'm 
tempted to believe the mechanical gauges. Should I try to track down a 
fifth gauge and let that be the tie breaker? Does it matter as it's all 
relative? Is 25 psi OK? Is there a way to adjust or calibrate an 
electric gauge?
AS always, many thanks.
Paul Michalczyk
65 Corsa Convert, born the 4th week of August, 1964