No way of knowing with certainty what engine you have without you doing more checking. In 74, the V8 of choice was the 350. That's what you may have, assuming nobody has changed the engine over the past 38 years (not that anybody ever does that)!
To know for absolutely sure what you have, check the numbers on the block: Camaro Decoding help on Chevy-Camaro.com

 

just for future info all small block chevy's of that era looked externally the same so without the block numbers it's really a guess, i have a 74 as well looks to be about the same color blue. i'm hoping to start a build thread shortly.
as for the mustang i really like them too! i'll be helping my buddy work on his 66 over the summer.
Dave.

 

you car looks awesome!!! i really like the light below the head lights. no idea what you call them:P

 

You're right he'll have to look on the block to find out for sure what he has. But with checking the VIN he'll know what originally came in it and an idea of what he has if takes the air cleaner off the carb and look at it.

 

O.o I really like that Mustang

 

The 5th character of the VIN is an H.

 

So stock it was a 145hp 255 lb/ft torque 2 barrel carb. 350-cc V8. Im fairly certain that its still the stock engine so I think that is all still accurate.

No Z28, but still fun! What are the modding options for these? I'm not used to the setup with the top mounted filter so I'm not sure what bolt-on NA options there are.

 

When you say you're not used to the setup with the "top mounted filter" do you mean you're not familiar with old style carb setups?

If that's the case, there isn't too much difference between carb and EFI when it comes to bolt on's. The biggest difference is EFI needs tuning to the computer with upgrades where else with a carb setup you need to have the right size carb and jets so it gets enough fuel and air. Just think of the carb as a combination of the TB and injectors all in one.

 

Great looking Mustang. The rough idle is probably from the gas sitting in the bowl of the carb and gumming up the jets. You can try some carb cleaner but if that doesnt work, I would suggest cleaning the carb by taking it apart.

 

by labored do you mean it has a lope? if it was his pride and joy he may have put a cam in it and that ruff idel could be a combo of bad gas and a lopy cam?