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.