69 convertible, no A/C heater core replacement

[Mike_from_NH]

Hi All,
Looking for some advice before I begin to rebuild the heater box. Don Rush has an excellent video on how to take apart the dash to do the rebuild/replacement heater box in a 69. I only want to do this once.

Different than the very simple looking AC heater core, one copper outlet runs straight towards the firewall, the other up and parallel to the firewall (above the heater core itself)… ( Heater Core - #18476-3 - National Parts Depot )
The heater hoses run through the firewall to attach to both behind the dash.

I would like to extend both thru the firewall, so the rubber hoses attach inside the engine bay instead of inside the dash.

Has anyone done this and if so, what components did you use to extend the 5/8" copper piping?
Any help on copper tubing PSI/temp requirements, fittings used, etc or other options much appreciated.

Thanks

[Calicat]

Following - been wanting to do the same on my 70 convertible without AC.

[LSR_Mike]

I did my heater core a while back and i DO have A\C. Could it be as simple as using a A\C heater core? This is my 69 engine side firewall, all the connections are on that side.