I believe he told you that so that you would get more Multi time. Also it will force you to become sharper in engine out procedures.
As a career move, everyone will have a different opinion. Personally, I believe that with your total time it will not hurt you and may help you. It will be all large multi engine time. Part 121 (it might be Part 125, but the regs are very similar to 121) time. Crew time. It all looks good to interviewers. While most if not all the piston pilots are gone from the airlines, you will be surprised how they seem to respond to people who have DC3, DC6 or Convair time. After all these were airliners at one time. And you will find that many of the systems and procedures will be similar to current jet airlines.
As for training. In ground school you will discuss systems, systems and systems. Large aircraft are systems aircraft. You will find the Convair systems to be much more complicated than todays commuter aircraft. You will also discuss performance. There is a bookful of charts that have to be dealt with before any flight. Yes, there are TOGA and quick reference charts, but from what I believe the operation you are talking with does, you will deal with the AFM charts most of the time. Other than Basic Indoc, that covers your ground school.
Flight training, depends on their approved training program. At the very least, you can plan on stall series, unusual attitudes, ILS approaches, non-precision approaches, normal TO, engine failure on takeoff, x-wind TO, x-wind landing, normal landing, engine out landing, missed approach and taxiing.
Hope that helps. Good luck.