6 to 8, use 8 or 9?

You'll need to swap the rearend just to have the 5 bolts patern and better axles. To determine if you need an 9" versus 8" the tires are an important factor. You can have all the power in the world but if you spin the tires instead of having traction then the rearend isn't that important, an 8" should do plenty. But if you install slicks, monstruously big or other sticky tires you'll probably destroy a few stock 8". Also if you have a manual trans. it put more stress on it, always depending on how you drive the thing. 300hp isn't so bad if you install decent size radial tires even with a manual transmission. I'll be surprised if you broke a stock 8" with your set-up. An exellent source for Ford rearends is Currie Enterprises they have brand new complete packages from drum to drum.

I have the 8" out of a 70's? Granada (same car I got spindles and calipers from) with no problems.... a little modifying of the shock plates was necessicary (slotting for bigger U-bolts) but the whole rearend I got for 50$

year isnt too picky as long as it has the falcon type suspension common to mustangs, falcons, mavericks, and granadas... some good pictures and swap info here the front wont require any fabricating, everything bolts on if you get the right parts... the rear you will have to get wider ubolts and elongate the holes in the shock plates in order to bolt to the leaf springs, either that or possibly the v8 shock plates had holes spaced farther apart for the wider axle tubes?.... but it is an easy modification, use the grinding tool of your choice, I think I used a grinding stone on an air grinder

also.... dont forget that the steering linkage has to be upgraded to the v8 type... parts are available from all the major mustang parts suppliers (you wont need outer tie-rods though....they "come from" granada (it's a good idea to get these from autoparts store instead of junkyard but you will be asking for the granada part, not mustang)... the whole granada steering setup will not transfer to the mustang like the brakes and rearend will

and... if you still have the "fruitjar" (it's cover looks like a mayonase jar lid)single cylinder master cylinder....deffinatly go dual cylinder, having one go out isnt fun, thank goodness noone was one the road was around when mine went out and there was a soft spot to crash. with the dual system if the master cylinder goes out you at least still have some braking power

(keep thinking of things)... and... dont think the article even mentions it but you'll need a porportioning valve to get the brakes balenced correctly.. and technically you're supposed to have a metering valve too (I havent installed one yet) the metering valve gives more predictable braking under light braking conditions