Take a look at a brand new general motors crate engine.
Go slowly and do your homework you have many options available to you. Keep in mind your block is a 19 years old.
Probably sea water cooled and pretty rusty inside. After you hot tank the block, it may be really thin and the problems begin. Also there are guys on offshore only that take out stock motors and install tall blocks fully loaded and want to sell what they have removed. Good luck and post what you do.

This is what I did to a mark IV last spring but you can do the same thing to Gen V.
1987, MIE/340 (7.4) 454 4 bolt main Big Block
Dip it and hone the decks and cylinders cut .060 over
Block was detailed with all casting debris removed
Scat 9000 Crank 4.25, 496 stroker
Scat 26385P - 6.385 in. Connecting Rods - Bushed -I Beam Forged 4340 steel
Howards Cams 120255 - Max Torque Retrofit Hydraulic Roller Camshafts, Hydraulic Roller Tappet, Advertised Duration 288/294, Lift .585/.610, 112 L/S
Howards Matching Valve Springs 98636 - 130# / 330#
Cloyes Gear 9-203 - Cloyes Wear Plate, 031 in. Thick,
Maxspeeding Hydraulic Retrofit Roller Lifters
Probe 496 Dome Top +18.0cc - SRS Piston Set
Clevite 77 Cam Bearings
Federal-Mogal 4400 M20 Main Bearings
King Engine Rod Bearings CR849HP
Cloyes true double roller timing chain
Mahle - Plasma Moly Coated piston rings
Melling 10778C (Anti-Cavitation) oil pump
Melling HV oil pump shaft.
New Harmonic Balancer
Rotating Assembly Fully Balanced
Comp Cams High-Tech pushrods #7954
Completely refreshed Chevy Heads 14096188 / 454 Open / 118cc / 3angle valve
APR head bolt sets
ARP 1.90 rocker studs
Norris S/S 1.7 roller rockers
Seal Power Gasket set
EDELBROCK BBC R2 AIRGAP INTAKE
Pro Comp 2752 Fuel Pump
Pro Comp Ignition
Pro Comp distributor
Super Stock coil
8mm Accel Hi performance wires
New single wire alternator
Mini (gear reduction and fully marine) starter
Edelbrock 800 carburetor
12" aluminum, oval air cleaner (with K&N filter)
Brass freeze plugs
Stainless steel bolt set
4” Stainless steel risers
496 MerCruiser exhaust manifolds with Tall Risers
Sherwood motor mounted raw water pump

You have a couple options. If it was me my main concern would be building the bottom end. You can still use the motor for now and change top end things while the motor is in the boat. Like heads, intake, exhaust, etc. etc. Build for your long-term objectives from the start even if you don't have the heads or other parts that would go with them.

f_inscreen name
That is a real real nice job you did !!!
What would you estimate, ball park you spent to do it less your efforts ????
I'm sure that is a concern for jmz

And I for got to ask you Who did the machine work ???
Thanks in advance for the reply !
Brian

Machine work was like 300 (floating piston pins cuts back on that) and balancing was like 125. I'm sure that there's a few bucks in it. My thing would be to Google search each part and then put in lowest to highest and find out who has each one part the cheapest. That's what I did.
The main point is I built the bottom end first. Still had the peanut port heads and some other things. Use the motor for a season while still looking for the others. And as I got each piece I just continue building on. But a bit of advice would be, now is the time to start looking in the middle of winter when people are trying to unload.
The reason I went this direction is because I knew what I had. In some cases it makes more sense to buy a complete motor (ie, 502ci) but you never know what you have until you open it up. And I would never buy a motor without thinking that it had to be totally rebuilt anyway.

fkn screen name, what rpm are you pulling on this motor while wot in boat? not dyno and what boat? length?

See there's the problem. You guys are not going to be able to compare numbers to my numbers. I have a 1973 Nova 19'. It's a big heavy and wide boat.
The motor I listed above, Volvo 270 outdrive with nose cone (with 280T trim), Solas 21 pitch prop. 5400rpm's @ wot = 63 mph
This is the boat,
[YOUTUBE]http://www.youtube.com/watch?v=qffe4Jt_fE8&list=UUgjMt8W_HOufy0AI1DgW3HA& index=10&feature=plcp[/YOUTUBE]

Hey finscreenname now that is a real nice job, kind of a shame to but it in the water !!!!!!!!!!!
I can see you put your heart into that project nice video.
Now its time to run the hell out of it.
Brian

I got ya Brian. And yes I have run the hell out of it. This one has been an ongoing project for about the last decade plus. The video is just the latest makeover. It's my hole the water that I throw money into. I never expect to get back even close to what I have into it. And I don't think I'll ever try. Hell that's like the 6th motor variation in it alone. I figure I'll be buried in it being that it's worth more than any coffin I will be able to afford.
More than anybody would ever want to know about it, supernova 19.com.
It is also the one that got me into doing restorations on these classics on a regular basis. I mean I would fix up one here or there, use it for a while and then sell it and buy another. But the 19 was my first Internet purchase ever and I used the Internet to find out its history and what it should look like during the restore and buy most of the parts. One thing led to another and recently Resurrection Marine.com was born.

Here's a little fun before the makeover and newest motor.
[YOUTUBE]http://www.youtube.com/watch?v=PfbzXetbUjE&feature=player_embedded[/YOUTUBE]

Man congrats on the make over cam out great you put alot of time and effort into it, looks like a blast..