The non-tach 67-72 gauge clusters already have an empty / blank opening where the factory optional tachometer would be located. And there are many alternative tachometers that
could be modified to fit that opening. Some will be easier to fit than others though.
The easiest way to go would be to simply purchase an OE style reproduction tachometer kit that includes a factory style tach along with all of the other parts necessary to install it into a 67-72 gauge cluster. These kits are available through a number of restoration parts vendors.
The next easiest would probably be to select an aftermarket tachometer that has a similar diameter to the existing opening. Here is a good thread showing the installation of an aftermarket tach in a 64-66 cluster -->
http://www.67-72chevytrucks.com/vboa...d.php?t=499570 The same basic procedure can be used with the 67-72 gauge cluster. However, there might be some clearance issues with the printed circuit depending on how far the tach protrudes beyond the back of the cluster housing. A possible fix for that would be to use the printed circuit meant for a 67-72 tach cluster (it is electrically the same as the non-tach printed circuit but has extra physical clearance for the tach).
Finally, adapting a factory tach from a different year range, make, or model of vehicle will probably require the most modifications. That is because the factory tachometers often tend to have mounting provisions, electrical connections, or other aspects that are configured specifically for their original application. For example, the 73-87 style tachometers mount into the cluster using screws around the perimeter of the face plate and a rear plastic mounting plate ... neither of which would come close to any of the tach attachment points in the 67-72 clusters.