Torque converter- The Riviera one will work, it has a lower stall speed than the smaller converter used in the GS. You can use a smaller converter from a 300 or 340 engine which will give you a higher stall speed. My GS has about a 2000/3000 stall speed in low/hi. The Riv converter will be a few hundred rpm less. A converter from a V6 will have an even higher stall speed.
The smaller converter has smaller diameter mounting bolt circle. Your '66 flexplate may have mounting holes to fit the larger and smaller converter. If not, you can drill holes to fit the smaller converter.
Engine mounts- Do you have the cast iron frame pads? they were 65-66 GS only. Repo rubber mounts are avail from the Buick Farm. You can make the adapter plates, as you know.
Radiator- Yes crossflow. The nailhead has the inlet/outlet on opposite sides compared to most GM's. You can either get a reproduction 65-66 GS radiator, or get a universal aluminum radiator. A universal for a mid-60's Ford- Mustang, Torino, will have the in/out on correct sides and fit in the Skylark radiator support. I've had good performance from an aluminum Griffen I bought over 10 years ago, but there have been a lot a lot of complaints about ones made after that. You can use a separate tranny cooler if you don't have one built into the rad.
Exhaust- There were some GS manifolds on sale on V8Buick. $250, which is a good price. Headers will cost much more unless you can find used ones. Ground clearance has been a problem on some headers especially if suspension is lowered or you have short tires on the front.
A repo engine wiring harness for a GS would be the easier solution. Don't know what is involved in adapting a 300 harness. The GS uses a junction block/horn relay on the inside drivers fender for the junction of the battery cable, engine harness power (alternator) and the feed to the main harness. You should add a fusible link between the junction block and the main harness as a safety feature, Buick started doing that in 1967.