I can't offer any advise on the 261 mount. But this may help you out, I have a 283 sbc in my 1950 3600 and I used the horse shoe mount from a '60-'61 v-8 chevy truck. The holes in the horse shoe mount match (visually) to the mounting system from earlier 6cyl AD truck front mounting system.

I just used the cover, plate, rubber biscuit and housing from an AD truck.

Tim

Tim
1950 3600 Some Pictures
1951 3100
1952 3100 Some Pictures