Are we talking about needing 2 inches for strength? I am not a machinist, but I was always taught that 1 to 1/2 times the diameter of the bolt was sufficient.
Yeah, in steel or cast iron that's ok.
Look at a steel nut. The height of a nut is not 1.5X the thread diameter. Max height is .337 per ASTM, or approx 90% of thread dia.
A general rule of thumb is minimum thread depth = 2X thread dia in aluminum.
If the OP is going to fill that depth of thread with a steel screw, it's not necessary. And good luck removing that fastener in 10 years.
Just looked at an aluminum Mopar Performance manifold in my basement and it has (2) .35 diameter thru holes and one .48 diameter thru hole in that location.
AND, my 68 FSM shows thru bolts inserted from the top with nuts under the exhaust manifold. NOT bolts going into tapped holes.
