I just got a single quad Offenhauser intake manifold and bolted it up this morning and found that it is about 3/8in lower in the front than in the back of the engine and I'm just wondering why it is not parallel to engine? This may sound like a dumb question to some but not to me. I'm not much of a mechanic so I like to ask alot of questions.

Because these engines sit in the vehicle at an angle and by the intake being angled, it makes the carb sit level relative to the vehicle once the engine is installed.

Stock intake is that way too.
Engine is at an angle, intake designed to be flat so it angles "up" compared to engine.

Good score, even on a stock engine the 4bbl will wake it right up.

Well I guess I didn't ask enough questions. I put this engine in a 46 chevy 1/2 ton and I installed it with the engine at a 3 degree angle. So what should I do now?

You could get a spacer and have it machined to offset it and make the carb level. Might be an easier way though.

Usually with engine swaps, you make mounts that let the engine sit with the carb being level once its installed, instead of a random amount of incline without knowing or test fitting the engine first. That may get you close sometimes, but I think the intakes are angled 5° or more.

Ford made a 1" wedge-shaped spacer for the early-1960s T-Bird (also has pipes to flow water thru the spacer, handy)
Theyre on Ebay, IIRC I sold one for $20 or $25.

Depends how far off it is. If its only 1/8" or so from level, just run it, you can set the floats a little different to compensate if needed.