I do pretty good with 7.4's for around $3500. Probably 450 hp depending on air temp, fuel and timing, but they are reliable 5000 rpm motors. I have the bottom ends checked out completely and bored for forged pistons if needed. Rework the rods (after magnaflux) and use the good arp 3/8"bolts. Make sure crank is straight and polished. Have rotating assembly balanced. I buy the Summit engine kits for around $460 that have everything (bearings, oil pump, seals with the trw 2399 pistons that have a slight dome). You will have to buy marine head gaskets. I have also picked up good large oval port heads (big valves)for around $500 or so that actually work better than the rectangular port heads for these engines. Inconnel valves are nice and I have a set on one engine but I havent needed them on others just like this. Edelbrock torker 2 cam and intake (most people dont like these low single planes but they work for me) and holley 780, chrome moly push rods, gold roller rockers (dont skimp here), full roller timing set (advance cam 4 degrees) Gil or similar exhaust manifolds and your off. It likes 10-14 advance at idle and 34-36 full. They start when you touch the key and will run at WOT all day.

Machine shop $1000
Engine kit $460
Heads $500
Holley (used) $100
cam $130
intake $100
push rods $ 55
rockers $250
Gils (used) $1000

Probably closer to $4000 by the time Im done. The crate engine might not be a bad idea but I like wrenching in the winter and there is some satisfaction from doing it yourself. Budman is right, the heads ,exhaust and cam are where the power is, the other stuff just helps it survive. His engine is very close to mine except I know holleys better than q-jets. Do a run with the stock flame arrestor and then with it off and you might be buying a new flame arrestor. Then get more air in the engine compartment too. I was amazed how much better it ran with the hatch open.

If you have the Gen VI do what Madmax did. Your speed is about 3.5 MPH slower than his was because you are only spinning the engine 4400 RPM. You need to get it close to the rev limiter @ 4700. 300 RPM is alot of speed.

If you have the Gen V (8 bolt steel timing chain cover) buy a set of Edelbrock oval port marine heads, an Edelbrock RPM Airgap oval port manifold(or the Merlin one), roller rockers(Comp Cams Pro Magnum SS), a better cam, and a set of Lightning Headers.

If you want to spend the extra bucks convert the hydraulic flat tappet cam to a hyd. roller. This will cost about 1000-1100. If you want to get by for less than 300 buy a good Hyd flat tappet marine cam. There are a bunch of people on this board who are using flat tappet cams with excellent results(Waterfoul, Dyno, etc.). Ask them before you buy. I doubt you will see a huge difference between the two with the power levels you are looking at.

The Edelbrock heads will raise your compression so don't even mess with new pistons. For insurance while the engine is out have a machine shop check your rods and mains for clearance and put new bearings in if needed. If you want replace the piston rings. Your bottom end is good to 500 HP so save the bore job and new piston money and put it into good exhaust(headers not EMI type manifolds).

BTW, double whatever you think you are going to spend! If you have a Gen VI, I have a Gen VI 454 Mag oil cooler to sell. You absolutely will need it. The 300-330 HP 454 cooler is way to small for this kind of HP. If you don't believe me just buy a $70 guage and find out for yourself.