If your sparky friend said to run conduit, tell him to get after it! That way you won't need to buy any pre-made parts. I am not against conduit (run lots everyday), just not a homeowner standard.
You will need to consider carefully how to extend the 2 cct that you need to move to the sub...
I am responding from Canada - our codes may be different (in general ours are more strict - but not always)
1) why run it in conduit?
2) Why 4 # 4 wires (is this 3 phase) if not 3 phase reduce the gnd wire size.
3) should be able to go to 1" EMT
If you need to keep all your wire sizes etc as...
Toss this out with the $4 you spent. Old FP is garbage (as is new IMO). Go to HD and buy a small new panel. I like Square D.
As an electrician in the Edmonton area believe what others are saying, the older FP breakers sometimes did not trip. Better safe then sorry.:beer: