Yeah, getting rid of those downdraft webers would help. I like and run webers but the 3 sidedrafts are the ones to have. My friend had a 72 240 that was mostly stock, except for a header and the distributor was converted to electronic, he had a set of SUs that needed a rebuild but decided to use the downdraft webers. They absolutely killed his top end, thing would pull decent up to 95-105 range then it fell flat on its face. I finally talked him into rebuilding the SU's and putting them back on. With the only change being the SU's he was able to pull harder up to 120-125. Thats not much faster but that's about all these cars will do stock, or close to stock. Alot of the limiting factor is low hp and torque, but the aerodynamics are not very good and make it even harder to go fast - I'm sure you could feel how much lighter the front end gets around 100 mph.
You can go alot faster is just takes time and $$$.
Don't get me wrong I love Z's but have always liked the 77/78 Z's the best. It just seems like you have a little more car under you, stiffer chassis, more torque, little less tinny sounding and mine have always been faster than my friends 240's.
Sparks out your tail pipe @ 135 - I'm not sure if thats good or bad, lol.
Sounds like something is really wrong...i think you need to re-tune everything...timing, check distributor, check fuel system,sync carbs, etc. my 72 will hit 100 in 3rd and all I have now is 8:5 to 1 CR, header, hotter coil and 0 smog stuff..
If everything is in tune and that's all she is giving you then do some compression testing and if that checks out too then you might want to try SU's if you still have them and see if it gets better.....
your spare tire well isnt full of lead is it???? hehehehe