Elizabeth,The words of Jesus did not stop after John 3:16 but John 3:16 lays down this basic truth for Christian life:(1) Eternal life is in Christ(2) We lay hold of eternal life and will not perish by believing in Christ.If we grasp this truth and this truth becomes a living truth in us, does it matter if it is "should" or "shall"?
Old or older English or perhaps simply British English uses "should" like "shall" so it is not necessary to even make a distinction.Here is an example from "A Christmas Carol" by Dickens:“Home, for good and all. Home, for ever and ever. Father is so much kinder than he used to be, that home’s like Heaven! He spoke so gently to me one dear night when I was going to bed, that I was not afraid to ask him once more if you might come home; and he said Yes, you **should**; and sent me in a coach to bring you. And you're to be a man!” said the child, opening her eyes, “and are never to come back here; but first, we’re to be together all the Christmas long, and have the merriest time in all the world.”In this context "should" means "shall" or "must" and so does the KJ version of John 3:16 that uses "should not perish."
This painting by Alfred Soord I think depicts the sentiment of the song in the OP- namely the "reckless" verbiage. This picture was in a children's bible ("The Bible in pictures for Little Eyes" by Kenneth Taylor) we had and I still think of many of the pictures in that book when I read the bible today.http://transformationsandwhispers.blogspot.com/2012/02/alfred-usher-soord-lost-sheep.html
I have been there too and known how faithful God is.Heb. 13:5 -- ……for He Himself has said, "I WILL NEVER DESERT YOU, NOR WILL I EVER FORSAKE YOU,"2 Tim. 2:13 -- If we are faithless, He remains faithful, for He cannot deny Himself.Christ’s love is boundless and purposeful.It does not fail.Even the winds and the sea obey Him.Can the cliff defeat Him?Our God saves.Our God is mighty to save!Ps. 145:3,5-7.21 –Great is the LORD, and highly to be praised, And His greatness is unsearchable.On the glorious splendor of Your majesty And on Your wonderful works, I will meditate.Men shall speak of the power of Your awesome acts, And I will tell of Your greatness.They shall eagerly utter the memory of Your abundant goodness And will shout joyfully of Your righteousness.My mouth will speak the praise of the LORD, And all flesh will bless His holy name forever and ever.
A modern hymn that gives Christ the glory:Come Behold the Wondrous Mysteryhttps://www.facebook.com/gettymusic/videos/10160019529160136/