Tubing is stronger because of the corners,, But I have use both for a crossmember.. Not as big as 3'', But have used both with no problems..So I'm putting a chev 350 into my 53 gmc pickup so now I have to replace my crossmember. what I was wondering is whats the difference between using a boxed plate crossmember to using a single piece bent 3" pipe?
I agree with some you posted but round tubing can be used very easy..Here's one I did using the round tubing.. And you can unbolt it if you needed too..For the trans you can buy nice prebent 2x4s ready to make plates for bolting in. Do not use round tube it will not give good bolting or welding surfaces.
For front engine cradle it should be boxed up. If not already may want to replace if its not a nice heavy junk of steel. That section of the frame should be very strong.
Trans crossmember is just to hold the trans and support.
The front member is structual and very important to how well your finish project will drive and handle real world conditions.