I am going to tub the 68, I have the frame work and suspension parts picked out but I dont know how to find the right rear. how do you determine what width rear housing to use or is it a standard size? the car is going to see street use powered by a blown 400 sbc, and auto trans, I am looking at using a 4 link set up. there is a 69 in the new "chevelle" mag that looks real sharp with 31x18.5 tires on 15x14 wheels I would like to go for that size tire and wheel set up. any input for any part of this process would be great thanks

Most guys that I know of that either did it or took it to a chasis shop always had the rear tire/rim size first.I guess they position them under the car,and then mesure in between.Just go to a chasis shop,and see what they say.
My 69 was already tubbed when I bought it,so I can't say more,I just know what some other friends have done.

If it's a 12bolt rear I had mine cut down to 47 inches. I did however cut 3/4"s off the passenger side to make the rearend center. So that way when it comes off the line it don't flex.

You need to know what wheel / tire combo your going to run, From there when everything is cut out, you put the wheel/tire assembly into the wheel well with the Body AT Ride Height both front and rear !! Very Important, Once you have them both placed where you want them and squared with the chassis, you measure between the wheel mounting surface and thats how wide you want your axle flanges. With that any Shop that Narrows Housings will know how wide to make the Housing you choose.

My 70 is tubbed and has 33x16.0 slicks under it. If I remember correctly it is 46" flange-to-flange. If you don't have a rear yet to narrow I suggest Ford 9 inch for versatility. The wheels on mine are 15x15 with 4.5 BS-draglites. I'll check my 68 as I can't remember the specs but the rear is out of the car so it is easy to measure. I believe it was narrower. It has 15x14 draglites and with 6.5 BS the tires sit too far in for my liking. I am planning on changing the wheels over to the 70 since there are street tires on them. The 68 I think is going to be strip/street so it is going to get the slicks. The 68 has the biggest tubs that Summit sells. Back-half is Chassisworks on the 68 and Morrison on the 70. Both are 4-link. Both were done by racecar builders.
Lastly my street tires are 33x19.50 Mickeys which look to be the exact same size as my beat up 32x14.50 slicks dimension-wise.
Good Luck with your project.