got a ? about my rear end!!!

well i want to put a tremec tko 500 into my 69 mustang mach 1 but i wanted to make sure there wasnt going to be any problems with the ratio in the rear end so my question is how can i find out what the rear end is on my mustang??? ill appreciate any info and thanks

Make a mark on the driveshaft and count how many times you have to spin it to make the tires spin once. Do you have an 8.8 or a 9 inch rear end?

i have one additional suggestion to checking the ratio of the rear end. It is correct to spin the wheel once and count the driveshaft revolutions, but to make it easier i block one rear wheel and spin the other two revs. It will give you a rough idea.