Usually, some have been known to double gasket to gain a little height. I ran a set of older rocker covers on my 302 with Twisted Wedge heads for a little to try and determine if a noise was just the rockers hittin my Fox covers...nah, it was a rod bearing LOL.
Matt
I have a pair of original valve covers that say "powered by Ford" on my 69 351w and I installed roller rockers. The only thing I had to do was turn the valve cover that you pour the oil into and smash down the oil splash guard, which was a poor man's solution of fixing the problem rather than spending $120 to $200 on new valve covers. The rocker arms don't hit anything, but it takes a long time to pour a quart of oil in it now. I just purchased a set of 1" valve cover spacers from Mustangs Unlimited to solve the problem of not enough clearance with roller rockers. Haven't received them yet, as I just ordered them a couple days ago. Not sure if they are a new product because I haven't seen them in the past. They are made from aluminum (if I remember correctly...or maybe it's plastic...can't remember). but I'm looking forward to trying them out. They were kinda spendy in my opinion for what they are ($89 pair). But you'll need an extra set of gaskets and longer valve cover bolts. They had 1 1/2" valvecover bolt sets for $20, but you can go to a local hardware store and buy bolts alot cheaper than that.
351w heads will fit on a 289/302. The only difference in the two blocks is the 351 is taller by an inch, but both blocks have a 4" bore. You can also use a 351w cam in a 289/302 block and visa versa. The only difference is the firing order. The distributor and intake won't interchange because the 351 is taller and wider. You can also bolt the 351w to 289/302 motor mounts. If you use the 351w heads on a 289/302 block, you have to either by special "step up" head bolts or buy a set of head bolt spacers/shims (can't remember what they're called exactly). The head bolt spacers slide over your head bolt and take up the space between the head and the head bolt. The 351w head bolt has a bigger diameter than a 289/302, hence the bolt hole in the head is larger than the 289/302 head bolt. The set of shims/spacers is $36 from Mustang's Unlimited. Just bought those also and am waiting for them to show up. Anyway, hope this helps you out....
I have a pair of original valve covers that say "powered by Ford" on my 69 351w and I installed roller rockers. The only thing I had to do was turn the valve cover that you pour the oil into and smash down the oil splash guard, which was a poor man's solution of fixing the problem rather than spending $120 to $200 on new valve covers. The rocker arms don't hit anything, but it takes a long time to pour a quart of oil in it now. I just purchased a set of 1" valve cover spacers from Mustangs Unlimited to solve the problem of not enough clearance with roller rockers. Haven't received them yet, as I just ordered them a couple days ago. Not sure if they are a new product because I haven't seen them in the past. They are made from aluminum (if I remember correctly...or maybe it's plastic...can't remember). but I'm looking forward to trying them out. They were kinda spendy in my opinion for what they are ($89 pair). But you'll need an extra set of gaskets and longer valve cover bolts. They had 1 1/2" valvecover bolt sets for $20, but you can go to a local hardware store and buy bolts alot cheaper than that.
Here's some advice: I bought a set of spacers off ebay for my 331. Best thing I ever did as far as controling the valve cover gasket leaks. No extra gaskets needed though. What I did was make a set of 1/4" X 2" studs from threaded rod. You'll want to get the highest grade numbered rod though as it's extremely easy to pull em apart from over torquing. Once you've made the studs, clean the old gasket junk from the heads. screw the studs in the heads. Then apply a bead of Permatex Ultra Blue sealer to the head flanges. Set the spacers down onto the studs and heads. torque the spacers down using flat washers and nuts. remove any excess sealer. Let dry overnight. Then remove the studs, nuts and washers and install the valve covers using gaskets and correct length grade 8 or 9 bolts.