Will a 92 siarra 350 motor fit in a 1986 firebird?

The motor will fit perfectly but you won't be thrilled with the performance. Truck engines have different cylinder heads and camshaft designed for low RPM "lugging" power. Truck engines have one nice thing going for them. They have a forged steel crankshaft with four bolt main bearing caps. You could "juice" it all you want with a better roller hydraulic camshaft from the 2011 GM Performance Parts Catalog.There are better cylinder heads available from Chevrolet for not a lot of money.