Reply to "Heater tubes dimensions - anybody have it?"

the original Ford outlet from the block (supplying hot coolant to the heater) was 5/8 inch OD tubing or ID hose, the return from the heater was 3/4 inch OD tubing or ID hose. No guarantee that's what De Tomaso used, but since you're engineering these parts from scratch, this is the opportunity to make the parts match each other.

However, I'd like to offer my 2 cents about a safer way.

The hot coolant supply could come from the block, 3/8 inch OD tubing or ID hose, the little heater in the Pantera surely doesn't need a larger supply than that. It could run beneath the car along side the radiator tubes, through the bulk head near the gas pedal and into the heater. No rubber hose within the interior, just copper tubing and flared fittings. Relocate the control valve outside the interior which will require a longer control cable and reducer hoses because its designed for larger diameter hoses.

The return line could use 1/2 inch copper tubing and flared fittings within the interior. Once outside the interior, the heater return plumbing could run under the car back to the coolant pump, or it could dump into the 1-3/8 diameter radiator return tubing near the point where the heater tube exits the interior. That would require a little welding on the radiator return plumbing for a hose connection at that point.

I had a terrible experience happen to me when an interior hose near the gas pedle burst while I was driving 6018. That has left me conviced that rubber hoses within the interior is un-safe engineering.

I'm not adament about this, I mention it simply to kindle thought.