I have a 67 Barracuda, and just filled my reservoir tank this week. I had pulled the back seat and rear inner quarters out because I also had to replace the driver's side cylinder pump or whatever you call it. (makes the top go up and down) Anyway, yes if you go through the trunk you can see the motor, sits in the middle right under the liner.
My fill cap was a slotted screw on the passenger's side of the pump. I took a very narrow four foot piece of rubber hose and attached a very small funnel to it. I held it high and gravity feed the fluid. Fluid for mine was Automatic Transmission Fluid, any brand with Dextron. Anyway when fluid runs out the fill hole that's all you need. Leave the fill hole screw off and run your top up and down three or four times to bleed the system and fill the cylinders. Fill your reservoir again, put the fill cap screw in, and hopefully that's it.
If it continues check the vert motor, or you might have a bad, or leaky cylinder. Check your carpet by your rear quarters in front of your back seat for red looking ATF stain or smell.