from the nastyz28.com site:

1965 WA 283 M/t - 175 2 C-10 to 35
1968 WA 307 M/t - 200 2 C-10 to 25
1969 WA 350 - - 255 4 C-20 to 35

Chevy used some suffix codes more than once. Who knows why since they have 12,167 combinations of three letter codes (omitting o,i, and q) and 529 two letter codes (deleting the same three letters). For the above example, the 68 and 69 block is easy to identify with the larger mains.

But what is a "conversion block"? Do you mean to bore out?