Want to humble myself and just put it out there I'm in kindergarten and wanna graduate at the end of this. I'm an auto tech by trade. Have a boat that I wanna repower. Want a mercury racing 500 or 525. Going in a 29 envision issue is can't find any that fit the budget. Built an ls mustang. Currently has a gen 6 7.4 mpi all cast internals. For starters will any bbc 1 piece rear main be suitable for the build?

There are others that know a lot more than me. What boat are you wanting to re-power? For the most part I’m betting the answer is yes, but like I said, what boat and what was in it previous?

MkIV 396-454 blocks are essentially the same - but some have 4 bolt mains. The 2 bolt blocks can be drilled/tapped for 4 bolts, but you have to line bore/hone it.
Early 502's used GM Bowtie thick wall blocks but by 2nd year they had non Bowtie siamesed 4 bolt 502 blocks.
Early Gen5 blocks are lighter in weight and thinner in some areas that make them less desirable. Those Early blocks had cast-in provisions for roller lifter spiders but they weren't machined for them. Later Gen5's did get the production roller lifters.
Gen6 blocks are stronger than Gen5's, they come setup for roller lifters, have cast timing covers with thrust provisions.
​​​​TALL-DECK blocks are also available in all "generations" as well as performance based blocks with stroker clearance and raised cam locations.
If you want to build your own, then your hp/rpm goals will help determine what you should start with.
A 502/509 is your best starting platform because you're at 500 inches on the stock steel crankshaft.
If you only start with a block but dont have a good crank, then a .030 over 502 with a 4.250" stroker crank gives you a 540. You can make a lot of pump-gas <5400 rpm power on that combo.

It's currently a gen 6 7.4 mpi. There is replacement for displacement. Wanting to just built the **** out of something and by next summer drop something in. Fully forged bottom with afr 265 head probably go carbed just for the sake of performance and more tunable. I'm on the hunt for a 500 engine. 525 would be nice but I'm a little at a time person not drop 15k today lol

If 525-ish is all you're after then you don't need to have a money tree.

Check out this torque-monster 468 build with AFR 265s.

The trick is keeping the squish clearance down around .036-.044" to suppress detonation at the 10:1 compression.

https://garage.grumpysperformance.co...id-dyno.11657/

With the AFR 265 you still will benefit from bore notching for flow on a 454.
A 502 (4.47 bore or larger) is large enough that notching doesn't help much

Realistically speaking .... what is your budget for a ready to drop in unit ?

Well I want to upgrade in stages but I found a long block no intake, exhaust manifolds. It's a 500..535hp long block. Plan is to keep it carb. So what manifold is best? And then stainless marine for exhaust manifolds. From there I'm going to run it for the rest of the year that way. Its rite at 8k for a fresh longblock. I guess a 500 is what block? 454 bored and stroked? Or is it a destroyed 502? **** if I know. Like I said this marine stuff is different. Long term 088 heads are what are in those engines? Long term do I keep them or go afr 265?

What the crap is a 500...535 ?
I'm not at all familiar with anything sounding like that. You need to get more details on exactly what this is.

You will never find a 502 block with "less than 502 inches". A "502 crank" is the same steel GM crank used in 454's. To destroke it would mean you would have to offset grind it and use thicker special rod bearings. Just not a likely scenario.
A 496 is a stroker crank in a 454 bore size.

This 500...535 may be a great setup, but it is likely a nonstandard collection of question marks regarding what's in it.

You've stated that you want to build something. It's best to START with a KNOWN collection of parts rather than some other random person's "build".

I sent a pm. So I don't look like a complete dip s*it