I'll ask my Firebird fanatic - he dropped a 200R4 in his 400 '69 Firebird a couple of years ago, built in his garage from an 87 Monte SS core. Darn good job with it, shifts hard and uses the lockout converter at his desired speed. He did a lot of research on this before he engineered (he is a GMPT engineer) it - looks entirely factory. Should be an easy solution -