No. They did have adapter plates to mount a V8 727, mid 60's truck. Hard to find but they are out there.
Pretty sure they are the small block pattern.So the A500 should work. The guys on the Slant 6 thread can help with that info.
As far as your converter question I don't think a 11'' converter will fit in the Slant's bell housing.
all ones coming up in my search for slant 6 / 904 converters are 10 -11.25 inch range . And stall all over the place . Bud not seeing what stock was . My guess 1500 ish ?