I bought my car with a 6 and swapped in my 383. Best thing I ever could have done. No regrets whatsoever. The 250 I6 sits safely under my workbench.
You will need all new accessory brackets, new frame and engine mounts, a bigger radiator, a new shroud, new hoses and belts, probably a new starter, probably a bigger battery.
The existing crossmember and driveshaft will work with a Powerglide, TH350 or 4 speed. The TH400 style crossmember is needed for an overdrive tranny or a TH400 and the driveshaft would need a new yoke for a TH400 and shortened for an OD tranny. You can reuse the generator and maybe the power steering pump. Front springs should be fairly close if you have aluminum heads, aluminum intake, and headers on your 350.
Hope this helps.
E85 racer and E85 carb builder
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68 Camaro, E85 powered 427" small block. 9.96 @ 133 MPH, 1.319 sixty foot on motor. 5.92 eighth @ 116 with a 1.42 sixty breaking beams with back tire on the bottle