Interestingly enough, drasty was sometimes misread/misprinted as drafty (that darned long s, no doubt), and it appears that way in some editions of Chaucer. In fact, Sir Walter Scott (mis)quotes Chaucer with that spelling in his "Essay on Romance" (1823):

But though the minstrels were censured
by De la Brunne for lack of skill and memory, and
the poems which they recited were branded as
"drafty rhymings," by the far more formidable
sentence of Chaucer, their acceptation with the
public in general must have been favourable...

...and when did the "correct" pronunciation take over?

from the OED:

(kɒŋk, kɒnʃ) [ad. L. concha bivalve shell, a. Gr. κόγχη mussel or cockle, shell-like cavity, etc. Cf. It. conca, Pr. conca, concha, Sp. and Pg. concha, F. conque (16th c.), formerly also conche. The earlier Eng. form was perh. conche, pl. conches, from Fr.: many pronounce (kɒnʃ, kɒnʃɪz). In L. the name was extended to other shells, as a whelk, a snail-shell, the shell-shaped Triton's trumpet, etc., and these senses passed into the modern langs.]

I've been saying "conch" wrong all this time too! Thank god I live inland.

The non-ch "ch" of the other big word in the title, "Christmas" suggests to me that the poet may be toying with us. Curse him!

in the crunch
i pronounce it conch

though i feel like a lunk
hearing conk

m.

Don't know how much to read into this but "conchs" and "conches" get roughly equal number of hits when restricted to English-language pages. (Without that restriction "conches" gets way more.) Here is a page with an interesting, mellifluous title and some nice pictures.

All the little boys in every version of the Lord of the Flies movies seemed to all be able to pronounce it correctly. That in and of itself makes the movie version unrealistic.