Wow you guys have got me thinking. Thanks, Alan Horvath,Carolina trucks & everyone else for the wealth of information and sites. Originally as a first time builder I had thought if I just re-installed the original mechanical fan that was on the 4.3 and the very thick 51 rad with shroud that it would handle the cooling. Because the fan on the 4.3 is much lower than the original 216 (about 2") and not up high enough I may have to consider a pusher fan.No shroud! (16",quieter 2000cfm with double relay wiring (to share the load)as was very well explained.)good photo's etc.
I presume the reason for a pusher fan is that it would not rob power from the engine like the mechanical fan would? Can be installed exactly where it is needed? Increase fuel efficiency? Also perhaps this would allow a little more room because of the removal of the mechanical fan?

Thanks to everyone for your time & consideration.
Mery xmas & happy New Year to all.

1951 Canadian Chev 1/2-ton Shortbox