1966 Plymouth Valiant Signet Hardtop

I see no one else was responding to this, so I thought I'd share my advice and opinions, and remember, you asked....
New to the site and looking for some guidance on a budget build. I recently inherited a 2 door Valiant from my grandma. It has a 2 barrel 273 in it. 3 on the tree automatic, bench seat.
Can I make the motor into a commando and add a edelbrock ld4b aluminum intake, layson’s full headers, 10.5:1 egge pistons, Holley 650 carb, and a Isky camshaft as close to stock?
You might be able to, but those modifications will likely cost as much or more than a good 360 crate motor, which will be more reliable, have more power, and allow you to keep the original motor all original, since that seems to be important to you.
What can I do about upgrading the k-member?
Why would you want to do that? You're are doing a budget build, not building a race car.
Who builds a affordable coil over suspension, these torsion bars got to go lol.
No, they don't actually. Torsion bars work very well and were used in very successful road race cars. Again, this is money you don't need to spend to just have a fun nice driving car.
The front end can use a weight reduction and use some tubular. Again, you are not building a race car. This is all just extra expense for very little actual return I would like to improve the steering, what’s the best manual modern day steering for daily driving and slight racing around. Rebuild the steering box in the car, it works fine for street and moderate performance use. Will 15X8 wheelvintiques fit without modification? I'm not 100% sure, but on a 66 no, I don't think so. I found a company that produces a fiber glass hood and trunk lid so I can reduce more weight there too. Again, why? The steel parts you have are more durable, likely fit better and best of all cost you nothing. It has a 7-1/4 rear end, not sure of the gears. Do I need sub connectors, if I want to make it a manual transmission will I need to fabricate a tunnel for the floor
Honestly, my advice would be to swap in a nice crate motor with a warranty, and rebuild whatever needs fixing. The result will be a fun, head turning, but relatively economical car that you can enjoy and won't take you two years and 30 grand to complete.