Theme: Po' Boys - Answers with the word HERO sandwiched within them.
20A. Coastal storm concern : BEACH EROSION
34A. Follow local conventions, metaphorically : DO AS THE ROMANS DO. (when in Rome)
41A. Get rich illicitly : FEATHER ONE'S NEST
55A. Deli lunch ... or, based on a word hidden in 20-, 34- and 41-Across, what each of those answers is? : HERO SANDWICH
Argyle and C.C., our hero. Two spanners today and two different religious climbers. Unique. Progressive split of HERO.
1. Cereal aisle regular __ Crunch : CAP'N
5. Classic Christmas stocking punishment : COAL
9. IRS submission method : E-FILE
14. "So sad!" : "ALAS!" and "Alack!" plus, 54D. Self-pitying lament : "WHY ME!"
15. Dance performed in grass skirts : HULA
16. Lag behind : TRAIL
17. Ready for harvesting : RIPE
18. Website for handmade goods : ETSY.com
19. Rooftop spinners : VANEs. Weather vane on Gary's house.
23. NBC skit show, familiarly : SNL
24. Enjoyed a meal : ATE
25. Checklist marks : TICKS
28. Prefix with sail : PARA
30. German automaker : OPEL
32. "Look at that!" : "OOH!". Got your "oohs and aahs" all set for the Fourth?
38. Web links, briefly : URLs
39. "__ out!": ump's call : YER
40. "Now I get it!" cries : "A-HA!"s
46. Org. policing Internet neutrality : FCC. (Federal Communications Commission)
47. Has yet to settle : OWEs
48. Play the lead : STAR
49. "The Family Circus" cartoonist Bil : KEANE
51. Cartoon shopkeeper : APU
52. Dawn moisture : DEW
59. Boxcars, in craps : SIXes. Presumed to resemble a pair of boxcars on a freight train.
62. Lay off : FIRE
63. Join forces (with) : ALLY
64. Halloween reward : TREAT
65. Steady stream : FLOW
66. Rock music style of the New York Dolls : GLAM. Hit the mute button and just watch them.
67. Rice field : PADDY
68. Julian and Sean, to John Lennon : SONS
69. Italy's Villa d'__ : ESTE. The Villa d'Este is a villa in Tivoli, near Rome, but there is a second one, in Cernobbio on the shores of Lake Como.
1. Dieter's count : CARBS. (carbohydrates)
2. Spock's father, but not his mother : ALIEN. Sarek is a Vulcan astrophysicist.
3. Castel Gandolfo holy retreat : PAPAL PALACE
4. Tiny amt. of time : NSEC. (nanosecond)
5. Speedy feline : CHEETAH. No, not that fast, not measured in nsecs.
6. Freakish : OUTRÉ
7. "Besides that ... " : "ALSO ... "
8. Air Pops chips maker : LAYS
9. Pierre's "And there you have it!" : "ET VOILÀ!". "Tada!"
10. Mali currency : FRANC. Mali still has a relationship with France, its former colonial ruler. Hence the use of the franc.
11. "Dr. No" novelist Fleming : IAN
12. Fib : LIE
13. Above-the-street trains : ELs. (elevated)
21. Lyricist with Rodgers : HART. They worked on 28 stage musicals and more than 500 songs. Another lyricist: 60D. Gershwin brother : IRA
22. Checklist component : ITEM
26. Vlasic varieties : KOSHER DILLS
27. Fizzy drinks : SODAs
29. Mgr.'s aide : ASST.
30. Twistable cookies : OREOS
31. Often blocked online lewdness : PORN
33. Run the party : HOST
34. Fanny : DUFF
35. Hoover rival : ORECK. (vacuums)
36. "Reader, I married him" governess : EYRE. Charlotte Brontë's Jane Eyre
37. Granny : NANA
42. Forthrightness : HONESTY
43. Pitcher in many still-life paintings : EWER
44. Cable station for game highlights : ESPNews
45. Small earring : STUD
50. Leading : AHEAD
51. Hank of Cooperstown : AARON. (baseball)
53. Flashy display : ECLAT
56. Does in, mob-style : OFFS
57. Farmland skyline highlight : SILO
58. Minimum __ : WAGE
59. Gas treatment letters : STP
61. Crossed (out) : Xed
Note: Today is C.C.'s birthday. How cool is that, to be published on your birthday?