Would a solid cam affect gas mileage verses a hydraulic flat tappet cam?
What is a good solid cam, for vortec heads, performer rpm intake, 3000 stall.

 

Cam spec matching cam spec, the solid will have "better" manners from less overlap. Moreso in the case of solid rollers versus hydraulic roller.

There is no real advantage in running a solid cam if you're not going to be going above 6500 RPM or so.

 

A solid lifter cam will be no better or worse than a hyd lifter cam based on just the lifter design difference. A solid lifter cam is capable of having a little more agressive shape to its cam lobe. (valve lift rate)
A good solid lifter cam for your 400SBC with vortecs a rpm manifold and 3000rpm converter would be Comp cams #12-678-4 (CS XS282S-10). ISKY cams #201027 (Z-27) would work well also.
Another good choice is Crane cams F-280-2 #114681
Either will make a little more power and torque than the cam you have now. fuel mileage will probabily be no better or worse as long as you keep your foot out of it.
If you want to switch to a mechanical cam one of these choices will work very well.

 

You can effectively get more lift with a hydraulic cam. A hydraulic cam is adjusted with zero lash plus some preload. A solid cam has some valve lash. If the setting is .028 lash then you're actually losing that much lift from the base circle to max lift. The ramps are different so you can't use solid lifters on a hydraulic cam etc.

Having a hydraulic and a solid cam with the same specs, fuel mileage should not change. Fuel mileage will depend on how much fuel the engine requires to make a specific amount of HP. If a cam swap produces more HP, then the cam is allowing the engine to consume more fuel to make that hp. If so then mileage should drop.

It's like a power adder. Pump more air into the engine so that you can burn more fuel. Burning more fuel produces more power. Burning more fuel reduces mileage. If the cam allows more air into the engine, it needs more fuel.

Of course a typical street engine rarely gets over 3000 rpm so using a cam that builds high rpm hp isn't a good choice for a street engine although if an engine produces more high rpm hp, it's also producing more low rpm hp. Now it gets tricky. You don't need low rpm hp. At low rpms, you want torque. Pick a cam, hydraulic or solid, that produces lots of low end torque for a street engine. You'll feel the acceleration better. These cams are typically known as an RV cam. For street/strip, you'll want something a bit better.