So Im fixing up a 1994 SBC 350 with forged crank and con rods, the piston heads are an aluminum alloy and flat topped. I'm gonna replace all the gaskets, bearings and piston rings, deglaze the cylinders and generally clean the block. I'm planning to swap the heads to Vortec heads, put a Comp Cams XE268H cam in, and replace the entire ignition, fuel and air intake system with Aces Joker master top end kit with the cast 92mm ram intake manifold with multi port injection. The motor will be mated to a 4l60e, stock since I can't service the trans myself. With a set up like this, how much power would you guys estimate id get out of it?

Probably similar to a typical miniram street build, 3xx hp 13.something type car. That 4L60E will likely be here for a fun time, not for a long time.

Would I be able to rebuild that trans to handle more power? New clutch packs or something? And what more would I need to do to get the HP to 400 round about?

The equation is
CID*RPM/3456=CFM*.067(warmair)=lb/min@100%VE

Thus
350*6000/3456=607CFM*.067=41lb/min (with 88*F air)
Now apply losses, example,
for 90% VE (41lb/min*.9= 36.6lb/min)
Drivetrain loss (36.6*0.86=31.4lb/min)
1lb/min=10hp , so
About 300 to 320rwhp dynojet at 6000rpm on a fully warmed up engine

At best, probably like this
600CFM*.069(cooler air)*.95(amazing VE)*0.88(amazing transmission losses) = 35/b/min = 350rwhp by ~6000rpm

Keep in mind
A. These assume the engine can actually hit 90 to 95% VE which means NO TPI and similar restrictive intake/head/exhaust systems
B. Drivetrain losses could be higher depending on tires, axles, bearings, transmission internal state, etc...
C. Forged internals generally add friction and reduce power output, they are often heavier and installed with less precision than a factory engine in terms of parts precision such as roundness and movement (larger clearances for example) which leads to more energy lost and more wear and tear

Forged internals for really best suited to forced induction applications, not a natural aspirated, in an SBC, and not a daily driver or reliable vehicle.
Once you hit the year 2002 a factory 4.8L/5.3/6.0L Truck engine with 140k miles + turbocharged will handle 600rwhp on factory internals, for example, for 200k additional miles if needed.

And the 05' version (L33 5.3L) will handle 800 to 900rwhp as a daily driver for 200,000 miles
Without forged internals and WITH factory parts is where you get the reliability from when tuned properly