This engine will be sitting close to the wall in my garage,from the manifold to outside there will be about 6 feet of pipe I will have to elbow the pipe out,would two 45 degree elbows equal one 90 degree elbow in back pressure

As long as the pipe is 3" ID it should be fine. The 3400 exhaust may be a bit smaller.

Two long radius 45* elbows is less restrictive than one 90* long radius elbow.

Hi Team,
Cat Dyno test specs for exhaust back pressure are :- Maximum for Naturally aspirated engines is 34" of water and Turbo charged engines is 27" of water.
We used to use about 12" of 1/4" copper tube to absorb the heat, tapped into the exhaust pipe about 6" to 9" beyond the manifold, before a plastic tube formed into a U shape fixed onto a board (Manometer) and read of the actual water level difference or a similar copper tube connected to an inches of water reading gauge works too.
Cheers,
Eddie B.