That 's a brew length of 70 litres
Taking into account a 5% beer that would require about 14Kg of grain and about 35 Litres of Liquor in the Mash.
That should be OK as long as you intend to flysparge, not Batch Sparge
In order to obtain 70 Litres once the boil has completed I imagine you would need about 85 Litres in the boiler at the start of the boil.
That will give you approx 8cm from the top of the wort to the top of the boiler so you should be OK.
You can always top up the HLT if you need to, but I imagine you will have enough, although it will be tight.
Don't forget you also need room to ferment this as well. In the coming months the ambient temperature tends to be a bit too warm for Ale fermentation so you will need to be able to keep it cool, A temperature controlled fridge is ideal, but you won't fit 70 Litres in just one. I know this as I have just upgraded to a 70L 50L 70L with the intention of doing 50 Litre Brews, but I only have one fridge at present so have a fermenter in my office, which is OK at present but won't be when it starts getting warmer.
Even though it takes 6 hours, it doesn't mean you have to sit and watch it for 6 hours.
The mash takes an hour, while it is on you don't have to do anything.
The boile takes an hour, once it is on you don't have to do anything aprt from add a few hops.
The cooling takes an hour, if you use an Immersion Chiller you can just leave it to it.