Lamb Shanks

So I’m cooking 2 7lb deboned lamb shanks on an open charcoal spit, i know i want to cook it to about 145-150, but how long it’ll take out in the open. Help?
Also, I’m cooking a 15lb beef rump roast alongside it…(and i’ve done this before) but do you think i’ll be able to sync up the cooking times of the these two meats?
am i in too deep? ah…help!

and thats 2 x 7lb shanks, not 27lb! sorry

7 pound lamb shanks? Shanks are usually about 1 pound. Do you mean legs?

ideally i’d like to just cook it really slow on the open spit. think i can do that over 4 or 5 hours? should i sear the outside before putting it on the spit?
yes, i’ll be trimming in before seasoning.

ya, legs