assume they are all seized.

try once on each nut to remove them using a 6 point socket AFTER you have rapped the nut firmly with a cold chisel a few times

if the nuts do not want to back off stop

you have two choices:

get them red hot and try again carefully

or

get out your drill with an assortment of 1/16, 1/8 3/16 inch drill bits.
start with the 1/16 and drill into the meat of the nut deep enough but not deep enough to drill the stud

move to the 1/8 then to the 3/16.

by this time you will have removed enough of the gripping surface most of them will back out easily. if not take your chisel and rap an edge of the nut as if you were trying to loosen it. this will often get them to easily loosen and they will turn.

force is not the way to go with this job.

if you break a stud off and there is at least 1/4-1/2 inch left here is a method i have used to secure the manifold to the stud without having to remove the stud.

http://www.rtlfasteners.com/Merchant2/merchant.mvc?Screen=CTGY&Store_Code=RF&Category_Code=R2

these can be bought in hardware stores, ... my local one carries this in 8mm same thread as the stud. once you have it use you bench grind to grind OFF the barbs to flat. get a similar size bolt cut it off so only enough is left to get 2 or 3 turns on to the t nut. now thread the t nut onto the stud and use the bolt head to tighten. you will get good gripping and darn close to spec torque.

there is sufficient space around the manifold holes for this type of thin wall blind t nut to fasten onto the stud.