ihave a 51 f1 iwas thinking of putting a 9in rear end but a friend suggested keeping original rear end This would be simpler but will it handle extra power of 351w Secondly can i change the original gears and still have some money leavt for other parts

 

Power isn't your problem - gearing is.

Even geared as high as it will go, the stock rear end will probably not get you much above 50 mph

The 9" is stronger and parts are more available - including gear sets.

I'd change it to the 9" and gear the rear end 3.0 -2.80 depending on what size back tires you are going to run.

 

The Princess is right...as usual! Gearing is the bigger problem. You don't want that 351 screaming at 3200 RPM at 65 MPH do you? The stock rear will handle the power okay as long as you don't abuse it but a 3.00 gear would be sooo much better...

 

im not sure what tires ill be running i put in amustang ii front end . im waiting for my rear suspension to come in from chasis engingering When i have that ill know my ride height i dont want to drag the front end on the ground. i was thinking of using 16 inch wheels but i wount mide any suggestions

 

What size bolt pattern did you get with the rotors on your MII front end? Did you ask to retain the 5 x 5 1/2" original bolt pattern or did you go with the more modern 5 x 4 1/2" Reason I ask is because you might want to think about having the bolt patterns in back match the fronts. The original back pattern is 5 x 5 1/2"

 

Two points. First, of course Julie is right about the gearing. I have a 9" with 2.75 gears equals 60-65mph at 2000 rpm. Nice crusing ratio. Second, be careful with larger diameter wheels in the front. If you have an F1,with the Mustang II IFS, The body will be lower and larger wheels might come in contact with your front fenders in a sharp turn. I have this set up and I couldn't go with as large a tire size as I wanted because of the contact with the fender. I used a 15" wheel. Hope this helps

 

thanks for the help i have the 5 41/2 pattern i didnt relize heidts offered thr 5 5 pattern .Thanks for the info on wheels i was thinking of going to a 16 inch wheel i guess you savr\ed me some aggrivation and money thank you

 

That will sync up nicely.

Go with 15 inch wheels (tires are cheaper too) and replace the rear end with a 9" that has the 5 x 4/12 bolt pattern to match your fronts.

 

What about the stock rear end with an AOD? Would that solve the gearing issue?

 

That would probably work well...

 

Use that stock rear if you can. We can help you with the install. Everything goes together much easier if they started out together. Ford eingineer things into engine/tranny/rear to make the best combo. They know more than we do. Mixing and matching drivetrain components certainly will work, however adapting is more work and money.

 

found a 9 inch this weekend to go with 351w and c6, Im not sure how to figure out the gears. I was going to keep everything stock until i priced out flat head rebuild it was going to cost between 8-12000 depending on how fancy i got. so decided for first build to make hotrod. Im putting best parts i can afford for anything thats weilded on . i have the heidts mII installed . next step is installing upgraded rear suspension with 9 inch. Then i was going to set trucks stance and install engine trans. After that a seat, steering ,gas tank,new master brake unit under cab then rad. Im hoping to get this all done before news eve working nites and weekends. I am very thankfull for finding this site, lots of good info and advice, if any one thinks i should do things in a different order id aprecaite the help. thanks

 

Your 9" should have a metal tag on one of the cover bolts. It will look like this (maybe not so clean):





The top numbers tell you what year and vehicle it's out of (mines a 9 3/8 out of a 66-70 Ford or Merc Passenger car). The numbers in the bottom left are the gear ratio in this case with mine it's a 2.80.

The 6CG 215 is the manufacture date.

Here's a table to find the number yours is from:

Ford Rear Axle Assembly Identification - Page 01 - FORDification.com

No Tag? Just turn the pinion and count the number of rotations of the axel shaft.

 

Julie,
my stock rear is a 373-i have a 302 w/ aod- in my 51 f1 i think that should be ok
i dont know what my rpms will be at 60-65 as i dont have it on the road yet

 

Here's an RPM calculator that may give you some kind of idea Tire size makes a bigg diff. For the sake of argument lets say you will use 235/75R15s on the truck (the diameter needed to fill the back wheel wells).

They have a roll-out distance of 88"

Gear Ratio Calculator

Assuming your gear ratios are:

First: 2.40:1
Second: 1.47:1
Third: 1.00:1
Overdrive: 0.67:1
Reverse: 2.00:1

The you will be going 67 MPH in third (direct drive) at 3000 RPM or 67 MPH in OD at 2000 rpm!