Thanks for those links. I had read most of them previously. The offset question is probably not one that can be easily answered, but I thought I would ask people who had originals to try to figure this out.
Maybe I should just take this one step at a time, but I figured if people were going to take the time to look at what they had it might be easiest to just get all the iinformation at once.
So for now...what I think I have figured out is the following for lengths. This is from AMA specs and some other Chevrolet sources. For 67 and 68 cars with the 396, different frame mounts for the engine were used which pulled the engine forward about 1/2 to 3/4 inch. Also note the holes for the 396 crossmember for use with a manual transmission had the transmission mounting holes about 3/4 inch forward. The PG, TH350, 3 speed manual and 4 speed manual transmissions are all close to 27 5/8 or 27 3/4 inches long (including bellhousing length). But the TH400 is slightly longer - around 28 1/4 inches.
67 and 68 Camaros used two different lengths. 49.96 inches for all applications except the 396 with manual trans which was 50.46 inches.
69 Camaros all used the same length, 49.56 inches. This appears to be due to the frame mounts all being the same for the big blocks and small blocks. Although I can't find anything listed, it seems the TH400 driveshaft should be slightly shorter.
So that is why I am looking for actual measurements, to confirm these numbers. If anyone has different information, please post it and list your source.