yes, use a sealant, get plenty around the joints after you finish, then turn the engine on and put your hand near the joints before they get too hot so you can see if its blowing or not, Use more sealant as required (try not to get it on the nuts!)
Now it sounds an easy job, but dont let it fool ya. WD40 your existing nuts and bolts in advance to fitting, have an angle grinder (Dremel would be ideal instead tho) just in case. Use new nuts, bolts (longer ones make the job easier if you dont have ramps/pit) new gasket wouldnt go amiss. Copper grease the new bolts before they go on, should be easier come MOT time.
Can be very un-hassle free, does depend on many things like: if its pitch black, pouring with rain, your torch batteries are dying and the the cat is half off but two of the nuts are welded with rust and rounded, your angle grinder is too bulky to reach the nut by the lambda you dont have ramps and the MOT is tomorrow. In this case id allow 4-5 hours. On the other hand on a good day and you had all the hassle last time so did all the stuff i mentioned earlier you can do it in half an hour.