Go with the '87. As much as I absolutely love the styling of the pre-80s trucks, the 87 sounds like a much better rig overall. You can always swap the front clip down the road if you want, not that hard.
87 pros:
-TBI (better mileage and more power stock than the carbed version, very reliable)
-454 (enough said)
-3/4 ton (better towing capacity, stronger frame, beefier suspension and steering parts etc)
-3rd row (I never use mine but it's nice to have the option and it sounds like you like the idea)
-Rear A/C (my '87 is front only and even way up here in NH it can't keep up with the heat generated from the sun beating down on the 47 linear feet of glass on a suburban, in FLA you might as well not even have A/C in a 'burb if it's not also rear A/C)
-Bigger mirrors
-Silverado trim package
-Nicer interior
87 cons:
-not as pretty as the 77
77 pros:
- In general the roundeye trucks are beautiful in comparison to the later ones, especially '85+
-On this particular example, I personally like the color scheme and wheels better.
77 cons:
-literally everything else. Much less power, lower trim level, 1/2 ton isn't great for truck stuff, not having rear A/C in Florida would probably kill you in the summer, and you said this truck has more rust than the '87