Great project! Glad you are finding the V65 Monza thread helpful.
The engine is a straight fit in the frame, but there are some details. Check the crank endplay before you swap because the crank thrust washers on the early V65s would wear quickly causing problems. All the small blocks have valves that have a reputation for dropping their heads at ~20,000 miles. The solution is to replace with new V75 valves and single coil progressive springs - they are a straight fit. (If the V65 engine has dual coil springs it almost certainly has original valves - replace both valves and springs.)
If you are keeping your transmission you will need to swap the clutch disk as the input shaft on the V65 is larger. The V65 has different timing marks (8* vs 10* as I recall) so you may want to swap the flywheel too.
You will need the V65 intake manifolds and carbs. The cylinders are 3 fins longer putting the top of the carbs higher and outward compared to the V50 and the V65 manifolds give more room for the throttle cables into the carb tops. Guzziology recommends removing the accelerator pump and drilling the atomizer larger and the mod works great on my bike.
The different intake manifolds mean you need to use the V65 airbox or pod filters.
The stock V65SP gearbox has higher ratios than the V50 'box and the Lario and V75 higher still. If your bike is a V50III it is an easy swap, but if you have a V50 (not sure about V50II) the output splines are different and you will need to find a V50III drive shaft (the V65 & 75 shafts are longer).
This is what I can remember now.
BTW Steam Driven on Wild Guzzi has put a V75 engine in a Monza frame but the cylinders are longer so throttle cable clearance at the top of the carbs is even tighter. Steam Driven was super helpful to me and has good information in the archives.