Considering purchasing a 71' D100 with the slant 6, automatic. I lurked here once before and thought I would ask some new questions specific to my goals.
I want to keep the truck as close to stock within reason, upgrade the breathing, ignition that most benefit daily usage with the hopes of some conservative performance gains.

I am told it will need a new fuel pump, read through some threads of mechanical vs electric, seems mechanical would be safer, reliable for my needs.
I see that upgrading to an electrical fan will yield some gains and efficiency.

Would upgrading the distributor internals to electronic from Pertronix be worth while and increase reliability?
Dutra header and free flow exhaust? Which carb setup would be recommended?

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1976 D100

I would use an electronic ignition distributor and a GM HEI module for ignition. Pertonix has proven less reliable than HEI when a good HEI module is used.

The stock exhaust manifold really isn't a bad piece when in good condition. Use 2 1/4" exhaust from the manifold into a high flow muffler. Muffler choice and tail pipe diameter depends on the sound desired.

The Super Six 2bbl carb and intake is what I'd use on an otherwise stock engine. If you can't find that and are handy with a welder you can modify the original intake to accept a Holley 2300 type or Motorcraft 2bbl.

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Joshua