Am working on a '61 235 mill for a bud, and asked to swap from hydraulics to solids for cam & lifters. Need to find out if the cam bearing, lifter guide, and timing gear mount sizes are compatible. Anyone have first hand experience?

Those parts are interchangeable.
Not sure what you man by "lifter guide"? The tappets ride directly in holes in the block.

You may need different pushrods, someone else will know that better than I do.

Thank you. Yeah, "lifter guide"s just a term for the lifter holes.

Use 56-62 261 solid lifters. They are the same dimensionally as the 62 hydraulic lifters. So no pushrod issues. Do not use the earlier solid side milk bottle lifters.

The timing gears are interchangeable from the 216 through the end of the L6 run 228 GMC-302 and some of the later fours. Some may have different timing marks, Stay away from the fiber cam gears. Try to use matching sets.