FYI ED, there is a 89-91 NA rotor listed on ebay with a buy now of $145 --
ebay # 7956735573, a fellow in Peurto Rico has 2 with a starting bid of
$190 -- ebay # 4543092448.  You can usually get a pair for $250 or less.
How about you finish your manifold development first so I can copy it....
;-)  Cheers and good luck with whichever route you choose. -- Ian