Necessary? Maybe not... but you WILL want them.
If you are serious about this swap, start shopping for fenderwell headers now. I would also look at the Schumacher tri-y's, they just might work, and would be better than any manifolds that could be made to fit
And if it was my project?..... Andy F's elephant ear motor plate would be my first purchase. I can be rubber mounted if necessary. I wouldn't bother. That just might give you enough room for under chassis headers or tri-y's.