Sounds like you have a set of 289/302 headers.

 

The headers are summit racing 6901-1hkr for 260/289/302/351. They went in and mounted without a problem accept for the floor board clearance. Thats why I'm hoping they have slimmer motor mounts.

I'm hoping for replacement mounts as I really can't afford the Ron Morris type, etc.

 

Hooker does indeed state they should fit your car. I remember a magazine editorial where the author mentioned how they odered a set of coated headers for a project car. The fit was impossible unless they hammered the headers and ruined the coating. They instead returned the headers. The manufacturer sent them another set that were identical and had the exact same part number. However the second set bolted right up. I think the point was that there just tends to be some variations in the manufacturing process.
Something to look at might be the transmission mount. If you drop the rear crossmember, the tail of the transmission will hang down to the point of almost touching the ground. You would easily be able to install your reducers. I suggest gently lowering the trans with a jack though. Going back up I'm afraid you will press the connectors back intot he floorboards though. A combo of shorter motor AND transmission mounts might help. Just a shorter tran smount might do it but you would be changing the driveline angle a bit. Whether enough to be a problem I can't say. Driveline angles can be tricky.
Surely someone else has some good ideas on this.

 

what heads are on it? I had the same problem on my 67 vert. Frankly I think hooker headers suck and their customer service sucks even more, headmans for a 69-70 351w fit great unless you have ported heads or afmkt heads as the ports are oval. I spent the dough and bought dougs headers d669-y headers, they are 1 3/4 3/4 length try y's and fit perfect.

my 67 vert http://htwheelz.smugmug.com

 

The heads are stock, factory ford.

 

they should fit, but as you can see they dont....maybe you got a mismarked set but like I said the ones I bought didn't fit either

 

The headers are summit racing 6901-1hkr for 260/289/302/351. They went in and mounted without a problem accept for the floor board clearance. Thats why I'm hoping they have slimmer motor mounts.

.

Engine mounts are the same for all small blocks,including windsors.
Headers that fit 260,289,302's are almost allways too short top to bottom to clear the floorboards with a windsor.Yes they fit the heads but that extra inch of deck hieght is whats getting you.

IMHO there is no such thing as a header that fits a 260,289,302 and 351w.(well a shorty maybe)
Now turn it around and you may be able to use a 351w header on a 302 but thay will hang low.

You need a header for a 69/70 Mustang with a 351w only.

 

I think I got the 6116 Super Comp headers.
Not exactly sure what the differences are, but they are also for a 351 in a 67-68 (Hooker calls them "swap headers" since this block wasn't available in these years).
I didn't have a prob with the collector hitting the floor, but I did have an issue with the reducer/outlet.
I ended up having to angle it down a litte to give the pipes to the mufflers a direct shot.