At first I thought it was just the oversized tires and lower gears, But when I get my 72 Ford F250 highboy up to 45 mph it is really taching the engine. I don't think it is shifting into overdrive. I believe it has the c4 automatic. And I remember seeing a post on here about that same thing. Any help appreciated...

 

There is no overdrive on your C4 or on any Ford auto transmission of that era. Ford's first auto transmission with overdrive was the AOD introduced in 1980.

You have to determine the rear end ratio and engine RPM at a specific speed to determine if the transmission is slipping. First determine if the fluid level is where it should be.

 

Yes I know it doesn't have overdrive, what I mean is I don't believe it's shifting into third gear. It only seems to be shifting from first to second when you put it in drive and that's it. It tachs right out at 45 mph with stock 4:10 gears. I do need to check fluid level and vacuum. I bought this truck in the middle of a frame-off restoration and the kid that was doing it did everything half-*** So I'm having to go through just about everything and do it right...

 

If you knew it had no overdrive, why did you SAY "I don't think it is shifting into overdrive"?? This is why people chase their tails and waste time because symptoms are not communicated well.
I wouldn't waste time changing fluid as this likely won't fix the problem. The filter is a rock catching screen. It isn't goiung to plug up unless the transmission is spent.
Place a drain pan under the tranny, just in case. Pull the vacuum modulator's vacuum hose off. It's at the rear of the tranny, above the pan. If fluid comes out, replace the modulator. It's bad. Then top off your fluid level as needed.
If that checks good, make sure the vacuum line going down to the tranny is in good shape.If all checks good, you need the aid of a good transmission shop, there's not much more you can do for it.