First step: compare circuit diagrams.
The BITX40 , if the VFO is disabled by removal or non-installation of the tuning coil, uses the oscillator as an emitter-follower (the capacitors at the base can be removed, and need to be for USB's higher VFO frequencies) and its succeeding stages to connect the DDS. In fact, only the last of that chain (not powered by the regulator that stabilises the VFO) is required - the DDS is able to be injected at either of these points, (ensuring there is a capacitor - as the Raduino has - so DC conditions are not disrupted). Your early version will have a similar arrangement, so that's your injection point.
Good luck!
