you can not mix and match lower plenums with different style heads. Must match like head/plenum together. The venturies are completely different between the 90-93 and the 94-96. The only want to get the new lower plenum to work is to either use the 10Y heads or port the flange area on the 40P heads to match the lower plenum.
Much easier to just use the correct plenum. If you need one I have about 25-30 on hand in my shop. $25