1. Tijuana setting, informally → BAJA
5. Many C-suite degs. → MBAS
9. Cry of resignation → IQUIT
14. Scads → ALOT
15. Ethel Barrymore was a great one to Drew → AUNT
16. Stage name for Milton Marx → GUMMO
17. Lead-in to call or taxi → ROBO
18. Indigenous Canadians of Labrador → INNU
19. Like a chill wind → NIPPY
20. Dead ringers → DOPPELGANGERS
23. Terminus of one BART line, in brief → SFO
24. Prop for a ball → TEE
25. Snagged → TOOK
26. They may be betrayed by stammers and blushes → SECRETCRUSHES
32. Tank top? → HATCH
34. The "weird sisters" of "Macbeth," e.g. → HAGS
35. Guitar, slangily → AXE
36. The "weird sisters" of "Macbeth," e.g. → TRIO
37. Part of a drum kit → SNARE
39. Perfect for a salad, say → RIPE
40. Purple-colored banknote of Canada → TEN
41. Boatloads → ATON
42. Some natural 29-Down → AFROS
43. Placeholders for favorite sites → PAGEBOOKMARKS
47. 1982 film inspired by Pong → TRON
48. Night school subj. → ESL
49. Vintage film channel → TCM
52. Someone comparable ... or what 20-, 26- and 43-Across may be, in different senses → THELIKESOFYOU
55. "Ave ___" → MARIA
57. Magnanimous words at a bar → ONME
58. Setting of many Winter Olympics → ALPS
59. Athletic shoe brand → ASICS
60. Scotch order specification → NEAT
61. Part of a scouting uniform → VEST
62. Blue Ribbon beer → PABST
63. Tailors → SEWS
64. On the briny → ASEA