There's 3 ways to do a BBC swap, you have to decide which is for you.
1. Mount the engine with the rear of the engine in the stock SBC location, usually with a commercially available side mount kit. This means you need to do some serious surgery on the firewall because the heads will hit it. You may need to modify the oil pan to clear the steering linkage. A 500 steering box might clear.
2. Mount the engine with a 3/4" forward side mount kit. You will have to do minor clearancing on the firewall adjacent to the passenger side head. You will have to modify the oil pan or buy a commercial one built for the swap. This scheme will clear a CPP500 or Borgeson steering box if the exhaust is done right. Clearance up front with the radiator and accessory drive is tight but works if you have the right parts and mount the radiator in front of its support (6 cylinder position).
3. Mount the engine further forward yet (1-1/4" - 2-1/4"). There are commercial mounts to do either. The firewall clearance gets to be ok. Oil pan to steering linkage is going to be a serious problem and that's where you might need to buy Ecklers rack if you use their mounts. Still is going to require a modified oil pan. The clearance in front becomes smaller and may restrict you to certain parts.
Header clearance can be dealt with on all 3. You can buy full length headers that clear a 500 box if you use #2.
The only way I would consider is #2, and that's probably the most common way, but the others can be made to work.