Spindle is not changed. The hub fits with same mounting flange location as original 4 lug hub. My understanding from talking with BCPP is that it is located the same as the original hub. There are also a user or two of his 5 lug conversion the the 6163skylark site. Everyone seems very satisfied. Ben is also available to talk with and is happy to discuss his conversion. It can take a bit to get hold of him. He does a 5 lug and also a 4 lug conversion. His website stinks.
I would go to a wider tire unless you are after a strictly show look. Also, 165 is a bit skinny for 6” wide rims. Lots of decent tires in 15” if you go to a 205 65 15. They also look good, handle well, and fit. Stiffer steering of you have manual steering. Go for something with an H speed rating for a better handling tire. Stiffer sidewalls and stickier compound. If you’re not concerned with that then don’t be concerned with an uprated speed rating.
You have a ton of wheels available in 15” 5 lug. I put the Wheel Vintiques 60s Camaro Rallye wheels on my 64 El Camino. You can put Moons on them or the Rallye trim ring and hub. Very period. Or go to lots and lots of alloys from different sources. Some very pricey, some very reasonable. OEM wheels may have the benefit of using some type of Olds hubcap..
With the 15” wheels, stock height springs, and BCCP kit you would be good with an aftermarket 15” wheel. Smoothies will fit. I use 1/4” spacers with my 4 lug wheels because the caliper fits VERY tight to the hub hat due to the small rotor. Some conversions hang the pad off the outer edge of the rotor. I worked to move it in for full pad contact and had that result. With the 5 lug i believe it uses a larger rotor with the caliper further from center so presents no wheel mounting face interference.