You can have a thicker mash, it is just a little more difficult to handle. May also reduce yield a bit, but not to the extent of reducing grain bill. Here is a little info on mash thickness that i put up on my site
Beersmith tells make mash in with 29 quarts if I sparge with equal that about 12 13 gal if you count absorption should you just boil down or will it work out?
That's still 14 gallons of water figure 2-3 gallon absorption should leave about 11 gallons of wort in the keggle boil off maybe 1.5 gallon 60 minute boil leaves me with about 9.5 gallons maybe do a 90 minute boil?
You will typically get worse efficiency when you are doing such a high gravity batch. You need to adjust your mash and sparge volumes so you get the right pre boil volume. Since the mash will likely be very thick, you should expect worse efficiency.
Beersmith tells make mash in with 29 quarts if I sparge with equal that about 12 13 gal if you count absorption should you just boil down or will it work out?
Why would you sparge with that much water if you want to end up with 6.5 gallons? Just sparge with as much water as you need to reach your preboil volume. Let's say you do about a 1.25 qt/lb mash. So you mash with about 29 quarts. You'll lose about 12 quarts to absorption, so that's 17 quarts in first runnings. You need 26 quarts (6.5 gallons) preboil, so then you sparge with 9 quarts to get there. So the mash is normal thickness, you just sparge less which will probably give you lower efficiency.