Thats great start! I just completed a very similar project last year on a 1970 Buick Skylark , originally a 350 Buick/350Trans , but now a 4.3L v6 and Nv3500 out of a ~2002 Blazer/S10 and 8'' of lift on 30x9.5r15s.
I bought the car as a "running driving car that only needs a fuel pump" which spiraled into me literally changing every major component other than the body itself, which also required significant rust repair.
A little advice if you're interested based on my build.
1. If the gas tank is even the least bit rusty spring for a new Holley ( or similar repro) as well as the " OE retrofit module " from Holley that puts the pump in the tank with a piece of hydramat - a little spendy but a good deal with how easy installation is- I did not do this at first and things went poorly with an external fuel pump and a cheap aftermarket tank .
Holley Sniper EFI 12-300 Holley EFI Fuel Tank Modules - 255 LPH - Camaro, Firebird & Nova
2. As you mentioned going to disc brakes in the front is an excellent idea - I have 70s style disc brakes front and stock drums rear and it stops almost like a modern car Note that you might have to do some fiddling with the brake lines depending on what kit you get and what proportioning valve they use. I had to cut up my prebent kit because of how things were mounted on my car.
3. Electrical system - make sure to check the wiring, cable control in this era of cars can be bad - the wiring harness in my car was a fire hazard as it had rubbed through in several spots. Eventually I redid all the wiring with a universal kit, wish I had done a modern OE replacement harness as my EFI harness was separate anyway .
4. (5 Speed swap hydraulics ) I don't know how it is in the X bodies but in my case the GM A body has almost 0 space for a hydraulic clutch under the dash. I fought with it for a while and finally broke down an ordered a malwood underdash kit which installed in about 30 minutes - and you don't have to mess with the pedals too much. I used a brand new throwout bearing kit as well just have to punch out a rollpin and change the fitting from a quick connect to a -3 AN line to mesh with the master cylinder as the stock lines are too short.
68-72 NOVA – Under-Dash Hydraulics Master Cylinder Clutch Pedal Kit (Billet Reservoir) - MalWood USA
5. Manual transmission choice- I used an NV3500 out of a 200-2002 s10 or blazer. this is the perfect length of shifter for an A body so its probably pretty close ( maybe 2-3 inches ) for the X body . If you are running less than 350 ish horsepower it should be ok , OD gearing isnt great at 0.78 but not terrible either . The stock shifter can be bad but there is a hurst shifter availble which is great ( I have one on my car). Best part is the NV3500s are cheap mine was ~150$ + aftermarket shifter. I would like a 6 speed but its hard to even find something for less than 1500-2000$ .
6. LS Swap - I basically did 3/4 of an LS swap- the 4.3 has a lot more in common with the old SBC than the LS motors mounting and exhaust wise. I only had to drill 4 more holes onto the back of the chevy pattern on the crossmember and then bolt on S10 4.3 frame mounts. You'll probably need a specific kit and exhaust for an X body. The only bit of adaptation i did was to cut and reflare the modern o ring style power steering line to accept an old school flare type fitting to mate the new power steering line to the old steering box.
Sorry for the wall of text , let me know if you have any questions ! This era of cars are special and deserve to be used and enjoyed.
