WWHD

ok i just bought a 89 351w block cheap!! and i have a set gt40p heads for my 306 i was building i was told that those heads on the 351w was a for sure 10:1 compression motor with good power is this true? and ive heard that you have to do some mods to the head to get the to fit on on 351w need help so i know where to go with my car thank everbody more help the better

S351R13

They will be around 8.8 to 1 compression. The head bolt holes will have to be opened up by a machine shop to get them to fit. They won't offer a big hp gain.

Indy

The gt40 heads if not modified, and if I'm not mistaken are pretty much equivalent to the D0E (1970) 351w heads. Having said that, they will probably leave much to be desired on a 351w, but offer improvement on 302's (306ci like yours). Check out the link below, to give you an idea of what these and other ford heads flow like. Hope it helps.
http://www.jason.fletcher.net/tech/f...a/castiron.htm
Indy

68CSTANG

The heads DO NOT have to be modified by a machine shop. The bolt holes simply have to be opened up with a drill to accept the Windsors larger head bolts. If you can use a drill, you can put 302 heads on a 351W.

mmm rice

i wouldnt put them on anyway, personally. put them on your 306 and build the 351.
302 parts=302 power

S351R13

They don't have to be by a machine shop but, I don't recommend doing it in the backyard. Most people don't have the tools handy to do it right. Sorry, but I am a little bit meticulous about what work I do. Thus my stuff lasts for a long time. Honestly most people on here looking for answers don't have the tools nor the knowledge to do it right. Thats why I give advice that won't lead them wrong.

WWHD

ok so im better off runnin the heads on the 306

WWHD

ok one other thing if i drill my heads to fit which i have a drill press and the skill to do this.... well i be able to still put them back on a 302 or not? after they been drilled

Ford1158

I ended up using a set on a 306 after I had drilled them out - I used the ARP stud kit for "302 w/351W heads" they use a stepped insert for the larger holes with 7/16" studs. It wasn't real cheap compared with other stud kits (about $135, if I remember right) but it's an option. I don't think I would use drilled heads without some type of insert to fill the gap - who knows, it might work OK but I wouldn't want to find out the hard way if it didn't.

Indy

If you open up the head bolt holes on the gt40's for 1/2" studs/bolts, you would definately have to buy a "351 heads-to-302" stud/bolt kit to go back to your 302 block.
Indy