Will 302 headers fit on a 400m block?

 

Not a direct bolt and go, but there are several workarounds to get them to work. One could buy 2V Cleveland header flanges and cut/weld them on the current header tubes(IIRC spacing should be close enough for this conversion). Alternatively, if your current header flanges have the material to cover the bolt spacing, you could buy a 2V header gasket set and drill the 302w flange to fit.

Now let's talk about the drawbacks. The 302 has less displacement, and 3/4 of the stroke of the 400. The 302 headers tend to be smallish on the tube diameter size, and they if they are tuned for a 302 will probably have shorter primary tube length than what's commercially available for the 400. Small tube headers can restrict top end of a 400CID assembly, however if it's a mild cam(approximately less than 210/210 @0.050", then you could get away with 1.5" inside diameter tubes...then we run into the tube length which plays on the effect on the torque curve of the particular set up. In most cases short tubes sacrifice low end grunt, long tubes promote low end, and should you have very short(block hugger) primary tubes, I'd definitely recommend an 8" collector extension to regain some low RPM drivability.

If you shop around, you'll find most 351M/400 headers tubes are offered in 1 5/8", and 1 3/4" primary tubes. Some entry level headers will have as small as 1.5" primaries. Remember the heads, camshaft, intake, carb, and exhaust should be suited for the intended purpose of the rig. Hope this helps.

 

. Also, the 400 is a TALLER, WIDER block engine, and the bigger exhaust port heads the headers attach to sit on top of that, so the header collectors may hit the bottom of your vehicle body... and other things...