Time machine questions...

I have 2 computers. Mac A (old one) Mac B (new). I transferred all my date from A to B. Then I did work on Mac B... Now I want to get all the data back on Mac A... a few problems.

I did a time machine back up of Machine B

1- I tried "migration assistant" to get the files onto Mac A, but it is telling me "not enough free space"
2- Booted from the CD disk of Mac B, but I do not have the original for Mac A, so it will not let me do a "restore from Time Machine Back up".

How should I proceed? What happens if I do a disk utility erase of Mac A. Can I restore from a back up or will that mess everything up.

Let me know your suggestions!

Thanks alot!!