Rather than figure out what You have why not figure out what You want ? If you are even remotely interested in more HP your going to put new Aluminum Heads on and a new Camshaft preferably a Hyd Roller and you will get the engine into the Mid 1980's Tech which has proven to work well. I can almost guarantee that the Truck Block has flat top pistons in it and it very well could be a 4 bolt main and a steel crank both of which are totally unnecessary for a street car anyway. Doing a Compression Check would give you some idea of how well the Rings are doing.
But since the Engine You have now seems to work I would be sorting out the Chassis and the rest of the car and worry about more HP last. We have not herd a word about the Big R factor as in RUST. It can make or break any project car. Sounds like the whole rear end assembly needs to come out which is typical for any of these cars. You say You might pull it out depending on how hard the Trailing Arm bolts are to get out. No you need to do it the other way around You need to pull the Bolts first and start your project there. If they are badly rusted, then what does the Frame look like, the Trailing Arms, what does the Birdcage look like and the Body Mounts ?

 

Here are some pics of various combustion chambers I took with the bore scope through sparkplug hole.

 

Things I'd like to do on the engine would be to bump the Compression ratio to around 10:1 and HP to around 250 RWHP. I'm just trying to figure out what I can work with, what to keep and what to replace.

From everything I've been able to check, the frame is in pretty good shape, just surface rust and everywhere I can get to sounds solid. I had the interior completely taken apart and the only rust on the birdcage was a small section right behind the passenger side visor, which I cleaned up and treated. So I think that's all a good place start from.
Yes, I'm sure now that the trailing arms will be need to be worked on. I can feel the rear end step out a little bit in hard corners.. I'll have to do a lot more research before I take that on though.. I'm starting to work on the front suspension now, so I'll move to the back after.

 

Those look to be 4 relief flat tops unless im missing something.. its kinda hard to tell. in any case a 4 bolt truck block is better than an l48.. I have a 76 truck block in mine that I built with flat tops and 64cc heads for 10.2:1 and around 375-400hp.

 

The Rear Crossmember Bolts and getting it to release can be just as tuff as the Trailing Arm Bolts. But once these are off its easy to remove the whole assembly in one piece. And this is the way to do it. When GM assembled them it was all done upside down. Thats the way I do it but I'm pulling the body off and flipping the whole frame and suspension with a forklift. There is lots of information here on the forum on how to do all this.
So now we see that the Block has standard Flat Top Pistons its just about as standard of a SBC as it can be and all the standard Hot-Rodding ways of doing things apply.

 

Your already ahead of a standard 2 bolt dish pistoned l48. Bolt on some modern 64cc chamber heads and an intake and swap cams and your good to go for an easy 300+ hp.

 

Well you got lucky!

Based on that pic, you do NOT have these very common late 70s style L48 pistons, with the huge D-shaped dish They only make 7.8 CR with the large chamber 75cc heads you do have. These are hard to get the compression up on even with better heads. You dodged a bullet by not having these.

Based on the 4 valve eyebrows, it appears you have have some flat-tops, something like these.

These 4 eyebrow pistons are also extremely common, and already make 9.0 CR even with the large chamber heads. With a Comp Cam 268H hydraulic cam (not the extreme one) you will have a great running engine, for cheap.

Your current engine likely has the ubiquitous 929 SBC cam, and would be like a 71 vette L48. So depending on carb, intake and exhaust you may have 270 HP now (gross) or 210HP net. That cam was designed for this engine and could add 40-50 HP. Say 310HP. (250 net)


GM data for 1971


If you wanted to go a step further, and add new aluminum cylinder heads, you could easily bump the CR up to 10.5CR, add a little bigger cam, and have a VERY strong running engine. Another 40 HP. Say 350 HP (300 net). It could easily run as hard as an LT-1 then.

I would not spend a dime on those heads, and would not bother with any other old head. On the new heads the ports and chambers are soo... much improved. Any new head would be a drastic improvement.

But I would do a compression check on the current package first, to see how good the ring seal is, before you do all of that.

 

Thanks for all the info. I'm not going to dream that's it's a stronger engine than what it is (it does run pretty good), but it seems there is hope it could certainly become one. I need to drop the oil pan to replace the gasket, which would give me the opportunity to see if it is a 4 bolt main.

Here is a better picture showing the whole top of the piston.



I dont know what pistons, combustion chambers and valves are supposed to look like, but do these appear to be in relatively decent condition? Nobody made any comments from the first batch I uploaded, so I'm guessing they are pretty ordinary.
Also, can someone explain to me why the one valve appears white and the other gray?
Thanks again!

 

Pretty sure it is a 4 bolt main being a truck engine.