With that amount of drugs and with the weights you are lifting gaining up to 255lbs shouldn't be a problem, whatsoever.
The key will be food. It's not the amount of drugs used so much as drugs+LOTS OF FOOD.
If you really want to bulk up like that then you need to eat and you need to pack in calories, period.
I'm 5'8.5" and 250lbs, but I'm nowhere near you in bodyfat. On the other hand I don't look that bad and I've put on 50lbs in the past 8 months using no more than 500mg test ew alone and a good half that time using only 375mg ew.
More anabolics means more gains, but without enough calories, enough protein, enough CALORIES it doesn't mean a whole lot.
The reason I gain so much, granted probably 25%-40% of the weight I have gained is fat, but the rest is solid muscle, is from eating alot.
It doesn't take a boatload of anabolics to get big if you are slinging heavy iron and eating alot.
There is no excuse for you not to hit minimum 255lbs with that amount of drugs in your body.
If you don't hit your goal you weren't eating enough.
Good luck Thatbloke, I know you can do it.