I am using this set up on 417 , I found that the heads with the Roush stuffers would ET slower than ported heads with out the stuffers . i do a gree that the spacers could be thicker to aline the ports better .What are you using for heads and pistons ? i am using 1971 heads and flat top K / B cleveland pistons with reworked 400 rods bushed to .912 small end. this gives me about 12,7 compression. Maybe the roush stuffers work better with lower compression.

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