i want to put the correct trans into my 1968 chevelle built in oct. the book i have says #s
39193390 or 3915079 for m20and 3915085 for m21
is there any book or sources to find correct trans i was also reading post about 5 speeds for
highway driving which i will be doing what about an overdrive addon thanks jim

The three numbers you have listed sound like GM part numbers, not casting numbers. The GM part number would have been stamped into a metal tag attached to one of the lower side cover bolts. A 1968 M20 recently sold on eBay that has the correct casting numbers you would look for, and it has an original part number tag, 3915076. These tags are usually missing from transmissions once they have been disassembled. Check out the auction:

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=4528529981&ssPageName=ADME:B:EF:US:1

And here is a link to the tag picture:
http://pic12.picturetrail.com/VOL424/525201/6635579/85932735.jpg

The assembly date for this transmission is listed incorrectly though. It was assembled in November of 1967, not 1968.

thanks bruce