Decision: 5x4 (SBP) or 5x4.5?

Question 1: In terms of functionality, either one will work fine. Your current rims are somewhat desirable so if you change them don't throw them out - someone will want them. If you want to go the completely authentic route keep the 5x4 pattern. In my opinion there is precious little value in originality of that level, but that's just me.

The big advantage of the 5x4.5 pattern is in wheel choices. You have a selection of SBP rims available, but nowhere near what you can do with BBP. Also, if you stay SBP tire choice becomes an issue. If you want 17" or 18" rims it's all but non-doable on SBP wheels.

Question 2: Another front disc option is the original Kelsey-Hayes discs. They are SBP but spendy as they haven't been made for a while and never were growing on trees. I can't help with the Pirate Jack option, perhaps someone else can weigh in on those.

As info, I have one each of SBP and BBP set-ups. I kept the SBP on my Valiant partly because by the time I found out I could have gone BBP, I was already heavily invested in SBP so I decided to keep it. I was able to find the rims and tires I wanted so I'm OK with staying SBP. However, I'm on the hunt for a 69 Dart, and I already have a pile of parts, including the 73+ front disc conversion to BBP, a new 8-3/4 in BBP.

Question 3: For most any kind of normal street driven car, front disc with a larger (10" or 11") rear drum should handle your needs. This is one area others may have input that differs. Rear discs are available, and as your performance level goes up, so does the need for braking. Fast cars need good brakes. Faster cars need really good brakes!