Using 400 SBC cylinder heads on a 305 engine
Can c/n 338882 SBC 400 heads be used on a SBC 305 engine?
You can- but with proviso's. The info provided here will hopefully help in the decision making process.
Compression ratio concerns
Swapping out a set of ~58cc chamber heads for a set of 76cc heads will significantly reduce the compression ratio. The 305 SBC came with 58cc nominal chamber volume heads for the most part. Using a 76cc nominal chamber volume head such as the casting number 338882 will reduce the compression ratio by about 1.75:1. This means that special attention will need to be paid to what the static/dynamic compression ratio is going to be when using the 76cc chamber 400 heads on the 305.
Milling the heads
Unless the heads are milled a lot and/or domed 305 pistons are used, the CR is likely to be very low- too low to support a cam with much intake duration at 0.050" lift/overlap. There is a limit to how much can be milled from a set of heads before the deck becomes too thin or the cooling passage by the spark plug needs plugged because it gets into the fire ring of the head gasket. Likely angle milling would be needed- and that means re spot facing the head bolt seats, enlarging the head bolt holes, cutting the intake flanges, paying close attention to valve train clearances and geometry, etc. Angle milling 0.100" will be around 64cc- still larger than the original heads.
If milling is going to be attempted, first zero deck the block. Then plan on using the tightest quench that's safe. From there the exact amount of chamber reduction can be calculated. A compression calculator comes in handy for this.
Generally speaking domed pistons are not as efficient as a flat top design. But that may be the only option to get acceptable CR when using 76cc chamber heads on a 305. When the added cost and loss of efficiency of domed 305 pistons is considered, it may become apparent that using an all together different cylinder head would result in better performance potential at less cost.
Also be aware that the 1.94" intake is all the 305 bore will take without excessive valve shrouding. If the heads in question have 2.02" valves, they will not work on a 305 even if the intake valve clears the bore due to shrouding.
The difference between a SBC used on a 400 and any other small block head are the "steam holes" between the combustion chambers. The steam holes were designed to rid the cooling system of steam caused by localized heating under some conditions due to the 400 block having siamesed cylinders (meaning the bore ID's are physically connected to each other due to the large bore size combined with the block having the same bore spacing as the 1955 SBC 265 engine).
There are quite a few builders who claim the holes are not needed on an engine that will see "normal" duty, i.e. not idled for extended periods like a delivery truck, for example. Among them is Joe Sherman (of Joe Sherman Racing) who says, "I have built hundreds of 400 engines, WITHOUT the steam holes, and never had a problem. Guys used to send me their heads for me to drill the holes, because I only charged 80 dollars to do the job, but I never did it for my own stuff."
SBC heads with steam holes can be used on other blocks without problem by using a 350 head gasket. Heads without steam holes can be drilled for them by using the correct 400 head gasket with steam holes as a template. This can be done by hand if attention is paid to angling the drill bit on the three holes on the intake side towards the exhaust side of the head. Drill the three remaining holes straight down, 90 degrees to the deck. It helps to start the hole straight down to make a small indentation to use to keep the bit from sliding as you lay the drill over to the correct angle. Steam holes can also be made using a drill press equipped with an adjustable table.
By now, it's clear using 76cc heads on a 305 is not the hot ticket. All one could hope to gain is a marginally larger intake valve (1.94" vs. the 305's 1.84" intake). Everything else is the same or worse than the original heads, including the port flow. It has to be remembered that the 76cc "smog" heads are among the worst flowing SBC heads of all time. There are guys who like the 305 heads enough that they will rework them for use on a 350 SBC. These same guys would NEVER consider using the c/n 882 head, period- even on a 350, where it would make better compression than on the 305.