Hi New to the board. Spent most of my life in preformance engines. 408,s are torque monsters. Spend the dough on a kit. I sold for Scat. The cast crank I beam rod kit is great. easy 600 hp level. 2 different blocks late and early. Deck heights are 9,48 and 9.50. 9,5 deck lets you square cut the block deck. Spend the money on a port job on the heads or home grow it. Get rid of the EGR lump in the exhaust port and open up the hole to 1 1/2 inches square. Make sure your exhaust manifold is matched.Don't go crazy on valve sizes. 1.95 and 1.5 valves will work. Just light bowl blend is plenty. Try to keep the pistons close to the heads. .040 piston to head is just about right with a dish piston. A Comp 268 magnum flat tappet hyd workd fine. Install it straight up dot to dot. Make sure the block is clearanced for the rod bolts. Easy home job with a dremel.Max 750 carb. RPM air gap. Get 28 oz balance. should work with your damper and flywheel.
set timing to be all in at 3000 and about 38 total. Shoul be a very docile tire burner that gets ok mileage. Cost is same as 393 but has more piston shoices. Hope this helps some.
Bob
BTW a 393 uses a 302 piston and a stroker crank with a 351 rod.