You haven't mentioned budget at all, so I will throw this out there. If Torque is the single goal, and you are OK with 4500 RPM's. Then finding an old 455 and doing a straight-up rebuild will give you the most torque per dollar. You can use stock heads, rods, crank, block. Some kind of mild cam and make 450 Ft. Lbs. of torque at 3000 RPM's and around 300 HP at 4500. Cheaply. If you want more and want to rev the engine to 5500-6000 RPM's, then a 400 block with stroker rotating assembly and better heads will be needed. Then expect 500 Ft. lbs of torque and 400-425 HP in a mild package at 2 to 2 1/2 times the cost.
My advise is to set your realistic goals based on what you really want to do with the car, then set your budget, add 25% for Misc. stuff you will run into, and then see if you can make all the pieces fall into place. Good luck. 66 Bonneville is a beautiful car. Seems like my choice #1 would best serve your needs.
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