I think you will have contradictory features here.....a frame cross section strong enough to hold a 1700 lb engine/trans will be pretty thick (tall). A 31 Buick had a straight eight (I think) but the frame was not anywhere near as massive as needed. The engine torque will also suggest a stout frame. Supposing you are going full-fendered, you have some limits as to what will fit under there......and how much underhood volume you have vs the engine size. I think you have a significant challenge getting the look right.....it cannot look like a 31 Buick on a semi truck frame.

You are probably on the right track to look for axles and other components from vehicles with similar weight engines. A Mustang II front end it way too wimpy......as is a late model independent suspension.

You need to take some measurements and get someone to do a drawing or CAD representation. You should also do some basic calculations regarding the minimum frame strength that you need.....you might get away with a boxed 6" tall rail with a third vertical plate....like two sections of 2.5 x 6 rectangular tubes welded together to make a 5 x 6 tall box....that would give you four vertical sections....OR maybe a really thick wall tube.

We have had good luck with having a CAD file laser cut for frame sides. To those, we weld the top and bottom straps to make the boxed frame. This way, the frame doesnt look like a straight tube.....it can match the original frame but be scaled larger and thicker.

Thinking about a 31 Buick...is the original frame wide enough for the engine and tranny? You probably are not going to get a wider frame to fit under the existing sheet metal......another problem.

Good luck with the project

mike in tucson