I also ordered from Hillbank a little over a year ago and went through all the same questions. 351/427, 302/331, and back to a 351/438. KeithCraft built the motor and Olthoff is doing the install. I am scheduled to visit Olthoff's next week for final fitting, dyno work and test drive. In about 2-3 weeks it will be in my hands in Southern California and you are more than welcome to pick apart what you like and dont like if it helps your build.
I went with KeithCraft for a couple reasons. I already have one of their motors with over 25,000 miles on it and never a problem. I finally decided on the larger motor with EFI because you can always tune the EFI for lower HP if desired. I also went with the dry sump setup, solid rollers with the Jessel valve train. On the motor dyno, they spun it up 7000 but found HP peak at about 6500 and a pretty flat torque curve from 4000-6000. If you go with someone like KC, I would suggest using Olthoff's serpentine setup as it fits nicely with the Dry Sump Pump, just make sure the pump is on the left side.
As for the car, I also went with the RHD standard body style with some of the "R" features from Olthoff. I went with the street roll bar as Rick has suggested, I'm 6' and sat in a "R" car and that roll bar was too close to my head for my liking. As for options, I had the rear wheels cut and widened, brake ducts, rear spoiler and front canards, electronic speedo and a couple other little items.
My plans are to drive it to Monterey next month for the vintage race weekend and numerous car events, went last year and it was a blast.
Good luck and enjoy the build!