My brother has an engine sitting outside his work, the heads are a 418 casting, couldn't find any info on mortec.com about them, the block itself has no number, but says GM Canada and 5.0 L on it. I guess it's a 305, but other than that I have no clue.

 

should have a casting number...it can be on either the right or left hand side of back of the block...usually if it has 5.0L or 5.7L on the left the casting number is on the right....

as for the heads, i think you probally misread the casting number, 6's and 8's look pretty similar, i think you have 416 heads which are 80-86 305 heads with 1.84"/1.5" valves & 58cc chambers...