that  can is part of the pcv system the hose coming off of it comes in too flavors open or closed the open system draws the fumes from the engine into the back of the carb via pcv the oil filler is open. the other system closes the oil filler off with a solid cap and get fresh air from the aircleaner as far as the heads go stick with the 283 i stll run unhardened seats on all my engines never hard a problem  and i do run them  hard the do need a little bowl work  but excelent heads comparded to anything post 1970