Here is my take on it and my recommendations. I don't see an easy way to add-on horsepower to get you in the 400 or over range for a number of reasons - in my honest opinion. Might be better off going with 3.90 gearing and take the hit on gas mileage and highway driving to increase your acceleration.
Assuming you are sticking with the factory compression and using racing gas or additives for higher octane.
You want to take into consideration that your engine short block is factory, cast pistons & cast rods. So no need to assemble parts that should be kept below the factory 5,100 RPM. Limit. Yep, you can spin it more, but you are always taking a chance of spinning a rod bearing at some point unless you know the condition of the rod bearings and have the better rod bolts like ARP's - or simply go to a set of aftermarket forged rods. (The Ram Air IV was factory rated at 5,500 RPM's with the same factory cast rods, but had lighter pistons and it easily spun 6,000 RPM's with its matching valve train).
Keep the factory Q-jet intake and carb as there is no need to go with an intake rated at higher RPM's when the factory pieces will do exactly what you want. Get the Q-jet rebuilt as mentioned and increase the primary jetting size to .074.
Not having headers will hurt breathing, so again, you don't want to build for higher RPM's if the engine can't breathe.
Get them hot tanked, magnafluxed for cracks, surfaced to get them flat (not milled down to raise compression), install new heater hose nipple & freeze plugs.
Install 7/16" Big Block ARP rocker arm studs to replace the factory "bottleneck" studs. You will need a set of poly locks to match as you want to "zero lash" the lifters when the time comes.
Install new bronze valve guides, but some like to go with the steel liners - depends on your machinists recommendations.
Some will recommend installing hardened valve seats on the exhaust side. I am of the opinion I don't need them when using Stainless Steel valves and good valve margins - but that is my opinion.
Install new valves. I like stainless steel valves and used the Ferrea brand valves. NOTE: Pontiac used 4 different stem lengths. I used the longer RA IV valves on my build which allows for a higher lift cam. You must use a matched set of springs for the valves as well as cam selected AND you will want to measure for the correct length pushrods (and I would get thicker walls on these) when using the taller valves. You can buy them from several sources, but here is a quick example: https://butlerperformance.com/c-1400...ce-valves.html
I supplied the valves to my machinist and then let him purchase the correct springs/retainers based on required spring heights and the cam lift I am using. I used a double spring versus a single and had Viton valve seals installed.
I did my own intake runner porting, blending, & bowl work, but not really necessary for a street engine. It is an expense you can save on unless you want to have it done.
What you do want, and will increase HP, is to get a 3 angle valve job ala Pontiac 3-angle valve job and not a Chevy 3-angle valve job. The H-O Pontiac blueprint guidebook says 15-30-45 blend for the intake and 30-45-60 blend for the exhaust.
Camshaft selection is almost endless. Each of us will have a recommendation and brand. Since you already have a high compression ratio, I would stick with a cam having a 114 Lobe Separation Angle. Tighter LSA's have a tendency to build additional cylinder pressure and not something you really want with your factory compression. If me, I would simply step up to the "744" cam which is just a little more than the "068" cam you should presently have and then use the 1.65 Roller Rocker arms to give you a better .447" lift.
Now I don't think you should have a problem
with using the 1.65 ratio rockers. The pushrod cup is moved closer to the rocker arm stud and what this does is move the pushrod in closer to the head. The pushrod goes through a hole in the head and when using higher lift cams, the pushrod can be jammed up against the hole and bend it - which leads to a big problem. The RA heads took this into account and the holes were elongated. Very easy thing to do with your heads off. The top of the holes are tapered wider at the top and narrowed down at the bottom. By "bottom" I mean about 1" down.
The cost to rebuild the heads is not inexpensive. I had my 7K3 heads rebuilt, less the roller rockers, for $1,100. If you really want to do them up, you might consider sending them out which would give you more potential in the future if you build your short block and go with something like a stroker kit. SD Performance- Pontiac Performance Specialists
Can you go cheaper? Sure. Do nothing with the heads and just add the bigger cam/lifters, and roller rockers as this is also an option at this point as well.
Once you get the engine squared away, the next thing to dial in will be your distributor timing/curve - which has been covered at length here on the forums if you do a search.
More options to think about.