I bought a 86 upper plenum and was told there are differences between the 86 and a 91 speed density upper plenum. I'm planning on using this 86 in my SD setup for my 383. Do I need to machine any vacuum ports or drill any holes? Specifically behind the throttle body area?

 

no, I used my 88 setup when I switched to speed density. I personally added a third vac port for the map sensor (just in front of the original two. (not required but, I felt better knowing it had it's own port)

the only time switching pleniums gives any problems is when using the 90-92 on a maf setup (sets off the ses light)

 

You want the MAP to have it's own vacuum port from the manifold if at all possible. I had a friend with a swapped MAF system converted to SD and it ran much better after I drilled/tapped a hole in the plenum for the vacuum feed. Before he had just T-ed a fitting running from the TB and it stumbled a bit at idle and just off idle at tip in...