The factory started installing instruments from the front in 1969 as access was impossible without removing the whole ventilation system. The '69 cars had a rubber sleeve that slides over the instruments and then the item would slip nicely into the dash. For the early cars there have been all kinds of methods used to install the later instruments. Mostly duct tape wrapped around the instrument until the right fit could be made. The problem is the duct tape does not always hold the instrument flush with the dash and removal to replace a bulb becomes a mess when the tape rolls up and the instrument is half in and half out of the dash.
Relief is on the way.
You should trial fit the instrument without the wires connected to get the fit right. If the fit is too tight trim off some of the velcro and if it isn't snug enough add some more. This idea came to me while browsing in a hardware store. I've been wondering why on earth it took so long to finally solve the problem. I have been using this method since last winter with great results and it is much easier than anything I have tried in the past.