help with this board selection

How heavy and tall are you?

Sounds like you need a bit more board when the waves get a bigger...this will give you more paddle power and allow you to get in earlier.

Changing the tail shape to a pin or rounded pin and tightening up the fin cluster will give you more control in hollow or larger waves.

I would be looking for something between 6'6 and 7'0 rounded pin with the wide point forward of center.. probably keep the width and thinkness similar to what you ride now

I'm no expert on board design but as tux said, the conventional wisdom is that less area in the tail and a smoother, longer rail arc (pin or rounded pin and a bit more length) will hold a line better in serious waves.
Also consider how the board is foiled (how the foam is distributed). More foam under the chest and more volume in general will help your paddling to get you into those faster bigger waves earlier.
I prefer a quad set up in bigger waves. Not 100% across the hydrodynamic theory here and hopefully someone else can elaborate but quads seem to get up to speed faster, paddle slightly easier and are less 'twitchy' (don't change direction as suddenly). I know lots of people prefer quads in solid barreling waves so there must be something to it.

Further to what the other fellas have said, if you're surfing 8ft Indo you really need to get some equipment to suit. Sure, some guys can get away with standard shorties but it requires sixth sense positioning and balls out courage because the lack of foam means you've gotta sit in deep.

Much better to get a board that's made for those waves. Without knowing your height, weight, or surfing experience it's hard to give specifics, but the general rule is to get something more drawn out. In guns and semi-guns you notice the elongation most through the rear of the board. So forget square tails or bat tails or anything that creates a wide tail block. You'll want a board that curves gently from the wide point of the board down to a rounded pin (or similar).

The reason shortboards have foam in the tail is to generate speed by pumping off the fins and lifting and driving off the concave(s). None of that is necessary in big waves, the wave will supply the speed so all you want is something that gives the requisite hold and release. Pin tails and rounded pins are the go.

There's enough foam in most guns and semi-guns to cover paddling needs, however if you're not a strong paddler then get one with a slightly bulked up forward section, meaning from under the chest to the nose. It'll get you up on top of the water quicker to reach max paddling speed, also the board wont tip side to side when paddling which slows you down.

All this is within reason, go too big or too thick and you'll be catching every wave in sight but unable to turn and make the most of them when up and riding. It's all about finding something that suits the curves and speed of those waves.

And if you need any other reason to pony up the cash for an Indo gun then just go pick up a sleek 6'8" from a noted Indo shaper. Put it under your arm, set it against the wall, pick it up again and rub your hands all over it. I'll try to avoid breaking into some breathless rhapsody about 'sexy curves' but tell me there isn't something utterly bewitching about them.

Stu your last line is spot on!

I'd have a whole quiver of pins and rounded pins if I could.
They look so sleek and sexy and amazing hahaha!