If you look at the words for the first line –
(1) How long, dear Savior, O how long
and the way they are set to music, you’ll see a potential disparity: the natural reading for (1) (as the beginning of a sentence of spoken English or as a line of poetry) has alternating accents, starting with a WS (iambic) initial phrase how long, but the tune begins with SW (trochaic). (Shapenote music has very strong accents on the first beats of measures; you can hear these in recordings of Northfield with its Isaac Watts text — from the Lookout Mountain Convention of 1968, here — and of the Rudolphized Northfield — from Jon Boden’s “A Folk Song a Day”, here.)
The Watts text has SW as a possible reading for the first foot of (1), with emphatic accent on how; the Rudolph text requires SW for its first foot, the word Rudolph. But in any case the tune requires SW. What allows the substitution of SW for WS?
This substitution (of trochee for iamb in the first foot of a line) is known in the trade as “iambic inversion”, and it’s probably the most common variant of iambic lines in poetry — a deviation from strict iambicity, but an entirely allowable one; lines with iambic inversion aren’t unmetrical. So hów lòng and Rúdòlph are fine beginnings for iambic verse.
Enjoy the music (thanks to Tané Tachyon for the Rudolph clip), and the gross disparity between the Watts text (with its joyful anticipation of death) and the silly Rudolph text.