There are multiple fairly economical ways to use the Ford Transmission on a GM motor. I have a 60's BOP bellhousing in my back garage with a 63 Ford Galaxie T10 bolted to it and another BOP aluminum bellhousing in a pile that is exactly the same.
As previously mentioned, that era BOP bellhousing is drilled for both early Ford and standard GM patterns. Read somewhere that the factory supplying the GM versions had a fire and so couldn't supply the GM versions and that's why the Ford bolt pattern transmission came in some 60's GM cars.
I also have a 327 with a Lakewood bellhousing on it and a later (67?) Ford Toploader with the wider bolt pattern bolted to that. The Lakewood has both narrow and wide Ford bolt patterns on it, so it will accommodate a T85 also. But, I think that version of the Lakewood is discontinued now and only available used.
Then while gathering up parts for my 60 Impala I also found an adapter bellhousing that looks like original equipment (front part may actually be a GM part...) for Chev motors on Ebay. It has only the early Ford T85 or early narrow Toploader bolt pattern. Not sure if it accepts the larger 168 tooth Chev flywheel size, but suspect it does from comparing it with a stock 348 bellhousing..
If you happen to do a lot of highway driving but don't want too-tall gearing, you may well feel the need for some kind of OD, as I do. So, I have hoarded at least 4 of the early Ford T85's with OD.
Read somewhere that BW produced a very few T85's for late 50's BOP too, but they are rarer than hen's teeth. So, if you want both a HD 3 speed and OD, I think the Ford T85 is the most practical option. Downside as also mentioned is, it isn't a fully synchronized transmission. But, it is rugged...
My 60 has a stick column, so at first I thought I would just leave that column linkage and use a mid-60's Chev 3 speed with OD (which I also scrounged up). Then being aware that Chev 3 speed (63?)is weaker than the T85, I thought maybe I'll just use the FordT85 with OD instead. A little linkage rod and front driveshaft modification and I would be set..
Then it occurred to me that I may as well use the T56, since I enjoyed driving my TA with a T56 so much and I was also curious how the 348 would perform with the abundant gear selections of that transmission.
So, now I also have the T56 rebuilt with a relocated shifter and an adapter LT1 type 153 tooth flywheel, but haven't completely resolved the starter issue yet, or the likely tunnel mod's required. Decisions, decisions...
Here is the adapter bellhousing that will accommodate both the early Ford 4 speed toploaders and T10's, as well as a 3 speed Ford T85... (Or the weaker Ford T86 for that matter..)