I have had a 351 Cleveland in my '81 for a couple of years now and have been tired of the poor performance from the stock manifolds after years of having headers on the engine in my Mustang. I finally bit the bullet and got the headers on and connected, they made a huge difference, much more power. The first thing to realize is while the Cleveland has the same heads as the modified and 400, is is a small block and has a shorter deck height. This means it will bolt in place of any small block (255, 302, 351W) but the exhaust ports are closer to the center line of the engine and lower. I had the engine in the truck and running but the manifolds would rattle on the cross member. This was resolved by adding spacers between the block and motor mount, I think I used 3/8" thick, but you have to make sure the fan does not hit the shroud or rad hoses. This spacing also helps the headers fit. I first tried 351m/400 headers, they hit the oil pan and hung too low. Next I tried the 1971 Mustang auto trans headers (Hedman #88320). The passenger side fit like a glove, the driver side needed clearance at the cross member, and a few spots for the auto shifter linkage. The headers fit nice and don't hang too low but the collector ended up almost hitting the transmission cross member. This was fixed by cutting the collector back and extending it up and over the cross member with an offset pipe on both sides. Then it was just a matter of fabbing the exhaust to hook up with my existing duals which are 2.5". It was not difficult to do the whole job but requires some pipe bending expertise. Overall I am very happy with the outcome, the only thing I would change is to fit everything up, tack weld it, and remove the headers to final weld. We tried welding in place and it was very difficult, moving the collector flange might help. One thing to note is with the longer offset collectors the driver side header will still be removable but the passenger side will not. It may come out if the motor mount is unbolted and the engine lifted slightly as it was very close.


Clearance required on drivers side




More clearance required on drivers side




Collector cut back. Pass side




Collector cut back driver side




This shows the amount I cut off




Pass side collector offset fitup




Pass side collector fitup




Both collectors in place




Driver side collector offset in place with new collector flange




Driver side collector offset




Driver side new crossover pipe 2.5"




Driver side crossover and pass side collector flange