I have a d2 td5 and I was just wondering if a tdci 6 speed box would fit? Has anybody done this or know if this could be done?

When driving my disco 2 td5 the heaters work perfectly and flow our good heat but as soon as I stop and leave it running the heaters blow out cold air till I start driving again. It's not losing any water or oil and runs fine. Would just like to get to the bottom of this problem!