Plumbing code does not prohibit poured lead joints however, with the modern materials there are very few plumbers today who even would know how to pour a lead joint. The last time I used my lead pot I was pouring fishing sinkers and lead balls for my cap and ball blackpowder rifle. It is a dying art. There are still some locations where no hub is not permitted and lead joints are still reigning supreme. Mostly large cities with a heavy union membership in the plumbing field.
You could also cut the pipe with a carbide grit or Diamond blade in a sawzall or a diamond blade in a angle grinder. This may limit the need to replace a whole section. A chain snapper may be used but often with older cast iron pipe it is not quite as good as you think it is and the chain snapper will crush the pipe.
Some areas may reguire the use of a banded coupling to join the cast iron and PVC pipes together. Check with your local plumbing inspector.
Also the connection to the septic tank may have specific requirements again check with your local inspector.
On another note I would not dig the hole any deeper than absolutly necessary to eliminate any settling problems that may occur. It is veryy important that the soil under the pipe stays compacted.