You are going to post pix of the final result, right?
(And if you and the wife are going together and want related costumes, she could be Billie Jean King to your Bobby Riggs. The Boomers would laugh and laugh – the kids probably wouldn’t get it though.)
Hmmm…shaved head, huh? Get yourself a lollypop, and be Kojack…
Or either a black or red singlet, with an evil goatee, and you could be Ivan Koloff.
I like it KOJAK
Or chain smoke cigarettes and be Kojack first season.
Get a grey Mao suit, a white Persian cat plush toy (top 'er off with a sequined collar), and be Ernst Stavro Blofeld. A monocle would be a nice touch, but not required.
Although if you’re going with a partner, I’d choose another that better incorporates her. For ex., the “Frankenstein’s monster” from *Young Frankenstein * could be accompanied by The Bride, if your wife can vamp it up like Madelyn Kahn.
Blofeld would be cool, but he is much more of a Sixties Character and too many people today would think you were dressed as Dr. Evil. 
Only one Blofeld Movie was in the Seventies, I think it was “Diamonds are Forever” and that was in 1971. There were four appearances in the Sixties and maybe 1 or 2 in the Eighties.
Jim
Then how about Martha Mitchell? You’d just need a dress, an updo wig and a yanked-out-of-the-wall telephone prop.
Is your wife shorter than you?
You could go as Sonny and Cher - with you as Cher 
Dr. Frank N. Furter from the Rocky Horror Picture Show. I’d imagine he would be instantly recognizeable to anybody from that era. Or, go with your wife as Brad and Janet–easy enough to do (just look like nice innocent young people), and uses your glasses.
Les Nessman from WKRP in Cincinnati: Balding and glasses. You just need a Band-Aid on you somewhere and to carry a trophy that you’ll call the Silver Sow Award.
Col. Kurtz from Apocalypse Now. Get some Army fatigues and carry the collected works of T.S. Eliot.
Other than that, I’d third (fourth? what are we up to now?) Elton John.
Hm. Shaved head?
G. Gordon Liddy.
And bring along some rubber [url=http://www.rotten.com/library/bio/misc/g-gordon-liddy/]rats to nosh on
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I love all the suggestions! So many to pick from.
FYI: Mrs. Spiff will not be in attendance … It’s a work-only function.
Then go as Mrs Spiff and express your regrets Spiff was unable to attend.
I’d say pick any of the Bradys from the Brady Bunch. I bet you’d make a smashing Marcia.
Mr. Furley (Don Knotts) from Three’s Company
Or Sheilds and Yarnell. 
My reccomendation would be go as a trooper from the Ohio National Guard , be sure to have a flower in your rifle.
White T-shirt, white pants, big smile–you’re Mr. Clean!
I thought of him too, or the Man from Glad. But I don’t know if either of them are identifiably '70s icons. They’re still making their pitches today, just as they have since forever.
Thinking of commercial figures though, what about Madge the manicurist? “You’re soaking in it… Relax, it’s Palmolive!” Or Mr. Whipple and his Charmin? Or did they cross decades as well?
How about Jim Jones? Sunglasses, “dry look” wig, safari suit, big pitcher of grape Flavor Aid (for authenticity’s sake), and you’re good to go.
Or, with the shaved head, you could go as Manson-trial-era “Squeaky” Fromme. If anyone shows up as Gerald Ford you can rub out the swastika, put on a red wig, and try and shoot them.
Or the Chevy Chase Gerald Ford would be an easy one – just wear a suit and keep falling over.
