If you keep all your existing engine accessories (timing cover, water pump, pullies, alternator, power steering pump) then any long block 1967-1978? 326, 350 Pontiac, 400 Pontiac, 428, 455 will bolt right up with three variables...engine mounts, exhaust manifold bolts on the outer head ports, and accessory holes in the ends of the heads. Engine mounts can be overcome by selecting a five hole block, a two hole block, or using conversion mounts. Accessory brackets are available but won't look stock. Using all the accessories that came with the engine is another option. Use accessories from the same year for '67, '68 and '69. Don't mix and match.

For the transmission, a popular modification is an overdrive automatic, it needs some light modification to work. TH350/TH400 are bolt in with the correct driveshaft and crossmember. What rear axle ratio are you running?