my first engine rebuild (318) advises

I've measured quite a few 318 heads and many are all over the place and some very tight with a seal, don't forget they only had to accomodate .390" lift.

Check yours to be sure, easy now you have them off and apart. When I did mine I established the height I needed and took a 5/8" flat washer filed to .120" thick, placed it on the spring seat with lube and when the cutter touched it stop.

It is actually too easy and I,ve done a few heads since the first which has paid for the tool. My usual procedure is to take them to my machinist for cleaning and inspection, have them planed and guides installed if needed then pick them up do some pocket porting port match and cut the guide, return for for final grind and assembly. Sometimes I'll assemble depending on what work is needed

That xe 250 h cam is smaller than I would want for a Duster. I put the 256 in my truck with a 3.55 gear and like it alot. Any bigger than the 256 with the small valves will lose some torque but gain lots of Horsepower.