[Jac Mac]

Mando,
While you could modify your current Radiator cap fitting, I would suggest you get somene to fabricate an assy similar to the one in the sketch attached and also purchase a header tank assy that can be remote mounted on the firewall. If you do this you will also be providing a source for your temp gauge /sensor fittings as well.

A Screw-On cap could be fitted on this assy @ top for initial fill purposes, but should not be necessary. As noted on the drawing the lower 5/8, -10 on the Header tank can be Tee'd into the heater return going back to the water pump.

Finally it would be wise to remove the 'goose-neck', in your Top Radiator hose by running it directly to the radiator to prevent the possibility of airlocks when filling.