Sounds like a cool build! I'll try to help
1- You need BBC motor mounts and stands, also they(stands) should move forward one hole on the frame rail.
2- Depends on what the headers are from/for. There seems to be a few issues with headers and manual trannys(clutch linkage) hitting the #7 header tube on sbc idk about bbc. I would imagine the same for bbc?
3- Radiator: bigger the better with a bbc especially, try to find a 4 row.
4- Saginaw will work great, untill you start it.
Really if you must have a manual trans you should start looking for a 4spd Muncie M20/M21/M22. or shell out the big bucks for an aftermarket OD 5/6spd trans.
5- Probably a little more squishy, idk there
6- 4.30 gears! well with no overdrive that will be some nasty rpm without overdrive. Cruising may be fun but you would be slugging along on the highway going 55 at 4k rpm. (depends on tire overall diameter of course)
Thats my 2 cents anyway...