'69 C10 307, with sbc 400 heads?

[w3lfd]

So, I bought my '69 last winter and finally got it road worthy, inspected, etc. after tons of metal work. Now I am finally digging into the motor itself.

When I bought it off of ebay, it had been listed as a 350. Upon picking it up and checking the block casting number, I determined that it was a '69 307 ( casting number 3956632).

I finally got to the tune up stage last week, and went to <insert copycutter chain store here> to get spark plugs. It currently had Autolite 25's in it. The auto store gave me the "wrong" spark plugs. This is when the box of worms opened up.

Pulled the valve cover and check the head casting number (3973493). I have checked a few places, and for the most part these are listed as early '70's 400SBC heads (76CC with 1.94 intake / 1.5 exhaust). One oddball site also indicated that they could have been used on 350's, but I digress.

Now the real question is this: How has this affected the performance of the motor? Is this a swap that was common, or am i dealing with a real oddball here?

It seems to run well, but I really have no frame of reference to determine. It is equipped with the original intake and 2 barrel Rochester carb. The truck does run quite rich, and fuel mileage is ridiculously low, I can only assume a side affect of oversized valves on a low compression motor.

I have ordered some 65k mile #416 Chevy 305 heads for the truck, as I had already planned on this swap. So it may be moot to discuss, but I am just curious what you all think.