I strongly recommend getting a rebuilt NV4500. I have been happy with Standard Transmission & Gear of Fort Worth, TX. The NV5600 and the later G56 are heavier, bigger, longer, and subject to limited parts availability. They are more expensive to rebuild, and the G56 has not enjoyed the reputation for durability that the New Venture transmissions have. You are getting into cost and time issues to attempt a conversion to a six speed. The durability questions of the NV4500 were worked out long ago and it is a good transmission. Standard can supply one with the upgraded fully-splined main shaft to prevent fifth gear from coming off, and with an iron tail housing that won't crack.