The early Ford trucks I did the 6 to 12 volt conversion on were 48 model F1-F5-and F7,so I am not all that familiar with the wiring color codes on a 51 model plus the fact that I have not done one in 10-15 years,so some details have faded a bit. The CVR's load capacity is limited,so it must power only the guages. You should be able to find one wire on the cluster harness that branches to connect to 1 terminal of each of the fuel,water temp and oil pressure guage,this is the only wire that must be connected to the unmarked output terminal of the CVR. As to the wire from the ignition switch to the circuit breaker being the one that feeds the guage cluster,I don't know. I never had any problems with blowing a fuse or breaker to the guage circuit,so I never paid much attention if they are protected by an inline fuse or a small breaker. I have no shop manual for that era Ford truck,so I have no wiring diagram to check,sorry. If you could get a look at a 51 Ford wiring diagram,it would simplify what you are trying to do.
I have no experience with the neat little solid state voltage regulators Bob posted a link to. I notice that they are limited to only.75-1 amp load capacity and the Ford site saya to not connect more than 2 guages to each regulator,so you would need to install at least two.
I know the old stock Ford CVR is a rugged mechanical device that is designed to power a full set of Ford guages and is pretty much immune to damage from voltage spikes and reverse polarity,somthing the solid state regulators may not be. As I stated earlier,the stock CVR has a bit lumpy voltage output,in some machines you can see just a slight movement of the guage needles as the CVR goes through a once per second, high/low voltage cycle. In general though,they do a good job for the intended purpose.