Well I have almost completed a 700r4 install. On my 57.
So first instead of having my old driveshaft cut and shortened, I found a new one on ebay and got it for 159.00 plus 9.95 shipping
It is the full drive shaft with both ujoints and 700r4 yoke, Now what;s really nice is that the ujoints are the same size as original ujoints,
So I have a 454 moved 3/4 inch forward with side motor mounts and a rear crossmember, I measured for the new drive shaft from center of ujoint cap to center of cap. and it was 50 1/2 inches allowing for free play.
When I installed the driveshaft today it fit perfect for me and the yoke was out 1 1/4 inch from the trans tailshaft. not bad guessing for a country boy.
here is a photo of the drive shaft and the ebay link
Next thing I had to do was to change my flexplate because I had a turbo 400 and the bolt pattern for the lockup torque converter was smaller than the turbo 400. the flex plate only cost 35.00 from advance auto. here is one on ebay for 36.00 and the link to ebay, notice it has dual bolt patterns so it will work on turbo 400, 350 turbo and 700r4
http://cgi.ebay.com/ebaymotors/BBC-CHEVY-454-SFI-FLEXPLATE-168-TOOTH-EXTREME-FP-454_W0QQcmdZViewItemQQhashZitem5636114de5QQitemZ37 0274291173QQptZMotorsQ5fCarQ5fTruckQ5fPartsQ5fAcce ssories
Next thing on the 700r4 I got a used one out of a truck and then had it built at the local trans shop, they installed an adjustable kickdown cable for me, I thought you could just leave it unhooked but I found out if it isn't hooked up and adjusted properly you will burn out the transmission.
Now for the speedo, It had an electronic speedo unit so I had him convert it to a mechanical speedo unit that way my cable just screw right up to it.
Now for the last part I ordered from summit racing universal lockup kit that do away with the need for a computer it cost 79.88 here is a photo and link http://www.summitracing.com/parts/TCI-376600/
Now one thing to note, Do not put oil in the new trans before you install this kit because you have to drain the oil out and remove the trans oil pan top install kit.
Hope this helps