This is an excerpt from http://www.crankshaftcoalition.com/wiki/Ford_144-250_inline_6_high_performance_building
Make certain you read the last paragraph.
Cams
Aftermarket cam grinds are available from Clay Smith, Comp Cams, Isky and Schneider. Be aware that some of the cams offered exceed the lift capability of stock valve springs, which is about 0.450". They also may exceed the lift rate at the maximum RPM. However, the good news is that there are a few aftermarket spring options.
Clay Smith offers a couple of options, but they may require machining the head for PC valve seals. Another option is Ford 302 intake springs, which will allow just over 0.500" lift and will provide sufficient pressure to prevent valve float at high RPM. These are single springs, which allow the use of stock valve seals. However, the valve retainers for these springs were designed for 11/32" diameter valve stems. In order to use them with 144-250ci valves, which have 5/16" diameter stems, it is necessary to mix the 302 V8 retainer parts and the 6 cylinder retainer parts.
Both of the 144-250ci and 302ci retainers have three components:
The outer retainer which contacts the spring.
The inner retainer which fits inside the outer retainer.
The two (2) valve lock halves which lock the valve stem to the inner retainer.
To use the 302 intake valve springs, start with 12 stock 302 intake outer retainers. Then take 12 stock 144-250ci 5/16" inner retainers and place them in the 302 outer retainers. Finally, you use the 144-250ci valve locks on the valve stem. Now you have a 5/16" retainer to mate the 144-250ci valves to the 302 intake springs.
Make certain you read the last paragraph.
Cams
Aftermarket cam grinds are available from Clay Smith, Comp Cams, Isky and Schneider. Be aware that some of the cams offered exceed the lift capability of stock valve springs, which is about 0.450". They also may exceed the lift rate at the maximum RPM. However, the good news is that there are a few aftermarket spring options.
Clay Smith offers a couple of options, but they may require machining the head for PC valve seals. Another option is Ford 302 intake springs, which will allow just over 0.500" lift and will provide sufficient pressure to prevent valve float at high RPM. These are single springs, which allow the use of stock valve seals. However, the valve retainers for these springs were designed for 11/32" diameter valve stems. In order to use them with 144-250ci valves, which have 5/16" diameter stems, it is necessary to mix the 302 V8 retainer parts and the 6 cylinder retainer parts.
Both of the 144-250ci and 302ci retainers have three components:
The outer retainer which contacts the spring.
The inner retainer which fits inside the outer retainer.
The two (2) valve lock halves which lock the valve stem to the inner retainer.
To use the 302 intake valve springs, start with 12 stock 302 intake outer retainers. Then take 12 stock 144-250ci 5/16" inner retainers and place them in the 302 outer retainers. Finally, you use the 144-250ci valve locks on the valve stem. Now you have a 5/16" retainer to mate the 144-250ci valves to the 302 intake springs.