I have used modern gauge internals in a '65 GTO cluster with good results. The key is to find gauges with the same sweep of the needle. Mounting wasn't too hard, hot glue gun holds very well with just a bit of flexibility. On a side note, I have been using a 30 gauge on my 250 for 23 years now, not sure how much pressure it has when running, but when hot at idle the gauge does come down some.
