My friend has two Olds 442, one is a '69 with a 400 and the other is a '74 with a 350.  I've been telling him for a good while to ditch the crap smog cam in the 350 and put one in that will maximize the mileage and use the '74 as his daily driver.  Right now both cars get about the same mileage (though the '69 uses premium), so my questions would be:

What cam would maximize mileage for the Olds 350, even at the expense of top end power (considering just about anything has to be better than a stock cam made in 1974)? It has 2.73 rear end gears.

and

What cam would make good power for a completely stock (carb to tailpipes) Olds 400 with out over camming the engine and perhaps reduce detonation?

Thanks.

I can't answer the question about the 400...as I do not have a big block.  But the small block I can.  I have a 350 olds with big block heads, performer rpm intake, 750 vacuum secondary carb, and various other goodies.  I'm running a cam with 204/216 @ .050 duration and .456/.484 lift from crane cams.  I pulled down 20 mpg with a 200-4R and 3.08 rear gears.  Idle is smooth and it still has good power...though that probably comes from the heads mostly.  A good exhaust system will also help mileage...as will headers.  A quadrajet can give really good mileage as well so long as you avoid driving too hard since they have such small primaries.  I know you only asked about the cam...but I'm just throwing some stuff out there.

Hope that helps a little

The stock specs for the 74 350 are 194/208 on a 110 LDA. Pretty good for gas mileage and torque. The problem with the 74 is the low compression. You could run a little bigger cam in it but you will lose some torque. Not much there to start with. The best thing to do is mill the #8 heads to get 69cc chambers and try to find some steel shim head gaskets. Advertised CR is 8.5:1 but if you measure, it comes out to 8:1. Milling the heads and using the thin gasket will get you to 8.8:1. If you get some earlier #5,6 or 7 heads they already have 68-69cc chambers and flow much better.If you mill those to 59cc you can get up to about 9.7:1. Then your cam choices can open up a little. In the end I would go with some small tube headers and a good exhaust system, gey rid of that restrictive EGR intake (use a performer intake) and with the 9.7:1 CR a 216/226 on a 112 LDA.
  As far as the 400 you shouldn't have any detonation. The stock cam is pretty big. 252/252 on a 110 LDA.You may want to try running the car a little cooler, make sure your ignition system is in top shape and the carb is in proper working order. Above and beyond that I would suggest replacing the stock head gaskets with a new set of Fel-Pro gaskets. that would lower your CR a little. Although what this does is makes the quench distance about .065 which can cause detonation but will lower CR. Ofcourse a bigger cam will bleed off some cylinder pressure but you might want to back timing off before you go that route.One more thing, no matter what cam you choose don't go over .500 lift unless you go with adjustable valvetrain.Also if you go with more than .500 lift you need to take the valve guide boss down to .700" so the retainer won't hit the vavle seals.It could also cause bent pushrods from the retainer not allowing the valve to open as far as the cam wants it to.

I don't think the cam is your weak link here.

I would uprage the rear axle, mill the heads for some compression and go from there.