As promised, I'm updating this thread with my experience so far.
Based on the great information I received, I contacted my local Part Source store (this is a Canadian aftermarket automotive supplier chain, part of the Canadian Tire brand) and asked about 7" round headlights for a 1972 Datsun 1200. They came up with a Hella Vision Plus 176mm HB2 Conversion Headlamp (part number 1A6 002 395 301 ). I ordered this light and received it yesterday. It fits the headlight bucket perfectly for diameter, however there is a 5mm gap between the back of the reflector and the shell. I will have to find a gasket material to fill that gap to provide pressure to stop the lamp from flopping about. The unit has a flat solid glass lens with very clear beam focus. It appears to take a standard H4 halogen bulb however it is supplied with a HB2 bulb for D.O.T. purposes. It is a very good replacement for the Slash 5 lens/reflector assembly. The only drawback to using this part is that it does not have a small bulb that can be used for a parking light. I will have to think about how I will solve that problem. I can post some pictures later when I have figured out the means I will use to secure the headlight assembly in the bucket.
Thanks very much to everyone who helped me.