hi Jon,
Sorry to hear about the unfavourable cholesterol result, that sure would be quite a shock. I'm not 100% clear if you mean that your total cholesterol is 340 or the LDL is 340? It's hard to know what's going on without the whole picture. Did your triglycerides also go up? What about HDL (the "good" chol.) If it's over 40, that's good, if it's above 60 it's highly protective, even over the higher LDL reading.
Unfortunately, using diet to correct blood lipids is just like using diet to treat diabetes. You must continue with it for life, because the risks for the disease never really go away. Diet and exercise are controls, not cures.
In Protein Power, the Drs Eades explain how someone who's been LC'ing faithfully might experience a rise in cholesterol and triglycerides. You can read about that phenomenon
here. In your case however, the problem seems to have been a rebound from falling off the wagon and adding back too many carbs too fast.
10 - 12 weeks/ 3 months should be reasonable time frame to see some correction in lipid levels. Try to focus your fat intake on monounsaturates (olives, olive oil, raw nuts, avocados) and saturates from unprocessed meats, eggs and butter. Omega-3's from fatty fish and flax will be helpful as well. Avoid polyunsaturates in margarine, soy and peanut oil and corn oil.
Monounsaturates especially will help bring your HDL up, which is very protective. I've personally improved my blood lipid profile by increasing my overall fat intake and focusing on monounsaturates. All the "baddies" have come down, and the "good" has come up. Because of disability, I'm not able to do vigorous exercise, so most of the positive results for me have been from my diet
Doreen