Bulkheads are walls, I think you are talking about the deck?
I would not use the spray-n-seal on the bottom of the doghouse but it may work well of the underside of the floors on either side of and behind the engine if you can get it cheaper than the Dynomat. If you can get to it the area above the transmission may be a great place for the spray.
A bag of blow in insulation and a broom stick suits that space better.
Rock wool is even better.
The spray foam is not your friend. It is basically half-set-gorilla glue and is too messy to consider (for me)
I bought two rolls of 4x50ft REFRACTORY CLOTH like they use on kilns.