[ck67goat]

That really depends on a lot of factors, such as what are your goals with the car (daily driver, street/strip, full race)? What are your HP goals and what final drive train and accessories will you run on the car. The '79 G body is a relatively light car, so you could not really go wrong with any of the 3 engine sizes since all will make decent power as long as you match engine components and build to go your transmission and rear end appropriately.
Also make sure you get the date and block codes before you choose and diligently research each block to see what exactly they are since you cannot make a blanket statement that a 400 block is stronger. Lastly, what is the condition of each block and the history of each. Are the engines currently running or disassembled? Are they original, rebuilt and previously bored out and crank turned? All these factors need to be taken into consideration to determine how much remaining using life is left in the engine as well as what will best suit your needs.

[ponyakr]

Yeah, as stated, there are LOTS of things to consider.

If you just want decent power to burn some rubber & not wipe out the Sag trans, then a low compression 400 is probably all you can stand.

I personally love to snatch on a Muncie 4-speed. So, for a decent pump gas car, I'd go with a Muncie & a low CR 400, such as a '78-'79 W72, or equivalent.

Would avoid any 400 block with a casting number ending in 557. Would also avoid a pre-'70 block, since they are not set up for the 3-bolt motor mounts. That eliminates all 428 blocks.

A 350 might give the Sag a better chance of survival. The '74 GTO engine is a good example. Had #46 heads. I like the 6x-4 heads. But most of the 350's which had those heads have the 500810 block, which is weak, just like the 557 blocks. Some say that's not a problem up to 400hp. Some disagree.

A 481988 block probably has the best resale value, of the common reasonably priced 400 blocks, since lots of guys use those for stroker builds. The XX 481988 blocks are the same, but do not have the holes drilled for the pre-70 2-bolt motor mounts

I'd avoid the '72 heads, and any others which do not have any end holes for the exhaust manifolds. I'd also go with heads which have screw-in rocker studs. Beginning sometime in May of 1973, all Pontiac heads have screw-in studs. These also have factory hardened valve seats, which are better for unleaded fuel. Some pre-'73 Hi-Perf heads have screw-in studs, but not hardened seats.

A good 455 will probably eat the Sag, and can break other stuff, if you get any traction. Probably not the best choice for a mild street car, with manual trans.

Just curious if your car has an 8.5 rear or the much weaker 7.5 ?