I have a new to me 1973 travco with the m375 chassis. I had planned to run a 413, but mine came with a 318 and a good 413 core has proved hard to find.
I will be building a 518 to go behind a 408 stroker. I don't know why it would need a 4k trans to live behind it. The 518's will live just fine without having to spend that kind of money. I am looking to see if someone builds a lock up torque converter with a heavy duty clutch though.