You can increase the amount of grain in the grain bill - but if you do that, you must also increase your hop addition amounts or else you will have a beer that lacks balance.
You should use any kind of brewing software (ProMash, BeerSmith, any Beer Recipe website calculator, etc.) to calculate the increase in gravity and bitterness - and you should try to increase the additions carefully while maintaining the same basic specifications. This is the same thing you should do if you have, for example, hops that are vastly different in A.A.% than what the recipe calls for.
To give you an example, I used a quick calculation on the website http://beercalculus.hopville.com/recipe
I inputted BierMuncher's recipe exactly as he wrote it in Post #1 for 5.50 Gallons batch size and using the AA% numbers he stated.
If you increase the Original Gravity from 1.048 to 1.065 - it throws off your bitterness quite drastically - the extra sugars mean that each individual hop addition adds less total IBU to the beer.
BUT, furthermore, increasing the hops to exactly the same
IBU won't work, because your Gravity Ratio will still be off-whack.
Here you can see what I'm talking about visually.
1. The original recipe.
Pay extra attention to these specs: the OG, the visual depiction on the sweet-vs-bitter graph, the Total IBU, the IBU for each single addition, and the BU:GU ratio.
2. The original recipe, with 2-Row increased from 7# to 12# - and NO other changes made.
(I'm ignoring the specialty grains for the sake of calculating bitterness
. If you want to replicate the flavor profile, the specialty grains should be increased by a small & corresponding amount.)
See how far out of whack the balance of this beer just went by adding all of that extra base malt? And look at the visual depiction meter - It's WAY off on the Malty end.
3. The new recipe again - but now, I have increased each hop addition slightly to get them back to roughly the same IBU Per Addition.
But do you see how the sweet/bitter graph is still way off of where it was originally even though we added some hops to it?
Simply matching the IBU is not good enough to balance the beer - You really want to find an approximation of the same BU:GU RATIO. This should bring our beer back into balance.
4. The same 12# recipe, but this time I increased the hop additions even further.
The general ratio of each addition to each other should be the same - so should the general flavor profile of the beer - but the total IBU increased in an effort to bring our BU:GU RATIO back into the right range.
(Remember: It's NOT going to be an exact clone, after all, it's stronger in sugar content! It's an entirely different beer, when you get right down to it! Just one that hopefully mimics
Does that make sense? I hope so! Enjoy!