Either of those engines would make great eye candy, but money is the main reason so many of us end up with a jelly bean SBC, they make great power for comparatively little investment.
I believe side mounts were offered as early as 1960, but only on trucks with automatic transmissions. This link shows the unusual early towers that were provided for those applications:
http://www.persh.org/pickup/perches.htmTo the best of my knowledge conventional stamped steel perches were introduced in 1963.
Both the Chevy and GMC frames were essentially the same from 1960-1962 with torsion bar front suspension. They shared a mostly boxed perimeter frame with a huge riveted and welded X crossmember in the center, much like that of a convertible passenger car (very beefy). In 1963 they switched to a pair of open C channel rails with conventional crossmembers. It was much lighter and more flexible and advertised as being tempered steel to make up for the loss of material. Other than a few dimensional changes this layout was used from 63-72.
The vertical height of the side rails increased one inch in 1963 compared to 1962, so a spacer must be added to the earlier frame if 1963 and up V8 engine mount towers are used. Here is a link showing what was required to install a 350 SBC in my 1960, just to show how the early V6 was mounted by comparison to your truck.
http://67-72chevytrucks.com/vboard/showthread.php?t=369663I don't have any experience with the V6 to V8 conversion in the 63 and newer frame. It may be that the V6 towers can simply be moved to the V8 location, or you may need the towers from a V8 Chevy. Hopefully another member with experience with the 63-66 chassis can provide a more precise answer.
