1.) Yes.
2 & 3.) Yes, the 261 and 235 use the same crankshaft and bellhousing, but...
The 261 makes more torque than the 235. A 10" clutch is the minimum size, and if you can find an 11" clutch it will fit if you have the right flywheel.
The starter will fit as long as the teeth match the flywheel.
4.) The pushrods should be the same for the same year (there were changes through the years, mostly in the rocker arms), as long as you have the same kind of lifters in both blocks. Solids and Hydraulics do not necessarily use the same pushrods, but the 261 and 235 blocks have the same deck heights and stroke. The bore is the differance.
I actually used my 216 bellhousing and starter when I installed the 261 in my truck. I had the flywheel re-machined to fit the 6 volt ring gear and used the 6 volt foot stomp starter. I used the 11" clutch flywheel that came with the engine that I got. It all fits pretty well.
The 261 is a great engine. It has noticably more torque and power than a 235. It also has better lubrication with the full flow filter. The rods are heavier too.
