The conversion depends on whose formula you use, but going by 85% for LME and 65% for DME you'd need 14 lbs of LME or 10.75 lbs of DME. 5 lbs would be too light for an imperial ipa.
[/I] Up Next - Hobgoblin
After That - Czech Pilsner
Primary - Humboldt Hop Rod (4/24)
Primary - NOT Wheat AG SNCA (5/5)
Conditioning - SNCA Clone (3/3),