Man. I've been reading through forums, google searches, online articles, and have reached the point of being overwhelmed and stuck spinning my wheels. To offer some context and background about me, this is my first resto project. I have a novice/intermediate level of mechanical knowledge and am blessed/cursed with an overly-analytical engineer's mind. That said... I'm looking to swap the TH350 in my 73 LT for a manual with O/D, but with so many options I can't figure out where to start. At some point, I know for sure that I want to replace the stock 2-bolt 350 block with a 4-bolt or an LS. I'll also be upgrading the rear gears from the stock 2.73s to a posi unit but again, what to choose? After playing with the Transmission/RPM calculator, it seems I need to figure out the transmission gearing before the rear and from reading other's forum posts, highway RPMs shouldn't be the only criteria for making a decision. For rear tires, 335/30x18's. What do you guys think? My goal is to build a car that I can take on the highways around town and have fun taking laps on track days. I need help narrowing down a direction to work towards. Thanks all in advance for your advice and for taking the time to read this and helping out.