I just finished doing mine and have a few tips for you.
You've got two problems and two options on the second problem.
The first problem is getting the hold down tabs on the fenton and offy to match up. The only solution for that is to grind the OFFY in some spots and the FENTONS in other. There is no way around this solution. I used a 4" angle grinder for this and it worked great.
The second problem is clearance between the intake and exhaust. The new Fentons tend to hit the offy on the back half.. There are two possible solutions for this. First, grind the bottom of the Offy to clear the Fentons, and maybe take a little off the Fentons too. If you go that route, you better figure out how you plan to heat the intake and put those pieces on it to make sure you take them into account for clearance.
The second possibility, and in my opinion a much better solution, is to take your fentons to a machine shop and have them mill the mounting surface at an angle so that the exhaust manifolds point a little closer to the block. That will take care of your clearance problem and I think milling the mounting surface is preferrable to grinding your offy or header tubes 'cause you won't risk making something too thin. Of course if you have the mounting surfaces of the flanges milled, you do that BEFORE you grind the mounting tabs to solve problem one.
Before you put anything together don't forget to take care of the two zinc plugs in the offy. They need to be replaced with aluminum plugs and heli-arced to properly seal it otherwise you'll either end up with a water leak or a Carbon monoxide leak, depending on how you heat the Offy.
Another good idea is to remove the studs at either end of your head, thread the holes, and use bolts.
Also, if you plan on having your fentons jet coated, get the clearance and mounting issues taken care of first before you send them out for coating or paint them.
