The best and quickest option is to go to your local s/s bender and have one made.You can search ebay for a nos or good backbox,but there were 2 bore sizes used. 45mm up to 1974 and 54mm from 1974,you can fit large bore to an early car.
Its possible but difficult to mate 45 and 54 together.
There is also a complete s/s exh on ebay at the moment for £400ish.
Maybe your local can make up a complete system at a good price,at least if it goes wrong they can fix it.
You can always buy a part or complete s/s system off ianp6man,he may even have some good s/h boxes!