Welcome to the chapter and to the world truck building. If you can I would get the c-6 rebuilt instead of the c-4. Or find out what was done to the c-4. If it was built for racing and has been beefed up it might be a better trans. In stock condition though if the motor was to much for the c-6 then the c-4 will not stand a chance. See if you can talk to or find out about the c-4. If it was with the 302 and they were planning on using it for racing then there is a chance it was built up and built right. Also by the time you get a shaft lengthened and balanced you will probably be close to the cost of the rebuild. The price you mentioned was not very high at all.