Firstly, welcome to the forum from KY. That's a pretty nice looking Monte.
You have alot of unknowns with the drivetrain, more digging on your behalf to find out more specs would help us to recommend good advice.
What are your plans for it??? Daily driver, street & strip, etc.
For a run of the mill sbc 350, if the budget is there....maybe look at a topend kit. That would allow swapping the heads, intake, cam, with the associated items to wake it up!!! Just as a reminder, if it's to be a cruiser or daily, stick with a dual plane intake to keep the powerband lower in the RPM's.
A set of headers to replace the factory manifolds is always a decent upgrade! I have a set of coated shorty headers for sale 😉. In addition to that, swap the single exhaust for a true dual, or at least a split dual system to allow the 350 to exhale- 2-1/2" is plenty big for pipes.
There are tons of other ways to gain some ponies, suspension upgrades to allow the car to handle which also allows the power to be transferred to the pavement better!
Others will likely chime in. Hope this helps to get the ticker working in pepping up that small block.