The fan clutches kinda sorta come with their own built-in spacer. Even though I have a 455 in my car, the OEM fan clutch for my year/engine placed the fan in the perfect position. So you could have an issue with the wrong fan clutch or fan pulley.

I had the same problem, but since my fan clutch was weak I replaced it with one from a Cadillac that was 1/2" longer than stock. Worked perfectly and is a lot cleaner look than a spacer. I had always been told that using a spacer on a fan clutch would move the weight off the clutch further forward and lead to early water pump failure. Don't know if that's urban legend or not. What salmon38 posted is another concern. Here's the website that I used to figure out the fan clutch I bought-
Hayden

Ok, I ran the numbers on the hayden site http://www.haydenauto.com/Online%20Catalogs-eCatalog/Content.aspx and used the "Parts Specification Search" tab (under the "part number" tab) and this allows you to select the different attributes of a given clutch fan. I pulled up the thermal clutch fan that is suppose to be for a 69 Firebird with A/C which is part number 2705 and copied all the part specs for that clutch fan and then went into the above mentioned search tab and started adding the different attributes and values for the 2705 clutch fan as I want them all to match with the exception of the fan mount height and overall height. Once those were all added I went to add the the fan mount height (I never touched the overall height as that would be determained by the fan mount height) and was only left with two options for the values. And that would be the standard 1.53 or 2.03 height. I selected the 2.03 height which brought back only one part number. Part number is 2707. This is used on a whole host of vehicles to include CADILLAC BROUGHAM (90-87)
CADILLAC DEVILLE (86-82), CADILLAC ELDORADO (84-81), CADILLAC FLEETWOOD (86-82), CADILLAC SEVILLE (83-81). It is also used on PONTIAC BONNEVILLE (87-70), PONTIAC CATALINA (81-70) , PONTIAC FIREBIRD (81-77), PONTIAC GRAND AM (80-73), PONTIAC GRAND LEMANS (83-75), PONTIAC GRAND PRIX (84-76), PONTIAC GRAND SAFARI (77), Pontiac GTO (73-72), PONTIAC LAURENTIAN (81-76), PONTIAC LEMANS (81-68), PONTIAC PARISIENNE (85-76), PONTIAC PHOENIX (79-77), PONTIAC SAFARI (69), PONTIAC TEMPEST (70-68), PONTIAC VENTURA (77). There are a lot more that this fan works on. So for what you need it looks like this is going to be the longest or tallest one you can get that will actually mount up for our purpose. I will wait to see if this part number 2707 is the same thing Desert68 used.

Thanks for digging into to this. Now I am wondering if moving the fan 1/2 inch inside the shroud with the 2707 fan clutch would make enough of a cooling temp difference to justify the expense.....

Why dont you go with the 2747 (heavy duty) that Desert68 went to? That is about 3/4 inch longer then the one you have now and you did say you needed about an inch more. That would get you real close and would tend to pull more air through, even if your fan blades where sitting 1/4 inch in the shroud, vuce not even sitting in the shroud!

Just to chime back in again, I can't point to the fan being 50/50 in the shroud as the magic bullet in cooling temps. I made the change when I rebuilt the engine. So I started with a clean and de-flashed block, did the water pump plate mod, have all the radiator side fillers, x panels and lower filler panel, clean 3 row radiator. What I will say is that after reading a ton about overheating problems and the fixes, I did all I could to not skip any steps. This will be my second summer with this set up on my 68 vert with a 461 and a/c. So far I have not seen it get above 185, even here in Phoenix.

I had to replace the water pump in my 69 last year, the fan before I did the work was way out of the shroud. So I did the mod on the back plate to pump distance. I used a 2 in. spacer to move the fan into the shroud, now the car runs way cool just sitting in traffic. Will have to check to see if anything is going on with that big a a spacer on the pump.