which plates?

[NITROUS]

I'd make the steel plates and use the mounting cups. You'll need to grind the raised area off the side of the cups that the plates mount to.

[86 Silverado LS1]

eBay Motors: LS1 LS6 CONVERSION MOTOR MOUNTS ADAPTER PLATES (item 230188741384 end time Nov-11-07 15:14:01 PST)

These are the ones I used (as best as I can remember). They worked fine. Keep in mind...once everything is in you cant see them. So as long as they are strong and line up they are as good as any.