Small block 727 and 904 transmissions will bolt up. As mentioned by the previous poster, a non-lock-up convert/trans combo is preferred. They (trans./converter) go together and do not interchange. But having a 340, if it's OEM with a forged crank, you need a neutral balanced converter. Cast crank 340's are externally balanced, as noted on the balancer/damper counterweight, and require a "340 only" externally balanced converter. Or you can use a neutral converter with the proper B&M flex plate for 340 cast crank applications. (LA 360's and Magnums use a different external balance.)
Lock-up trans/converters started in the late '70's. Converter may look a bit different, but I don't know how to tell if in the car. Lock-ups have about 3/4" of the input shaft tip machined smooth.
727 and 904 are also different lengths requiring different driveshafts and there are a few different flexplate/converter bolt patterns that you may need to match up if swapping parts around.
