Back to the drawing board... stock low dollar 360

As for cheapest stuff - I'd do a set of used cheapie headers, and buy a hole saw set and put it to good use. Stupid things like cutting the un-used portions of the front end bolts off will get you faster for time invested. plus the headers are lighter, and will help the 360.

I am guessing you like the gearing, and the convertor will not do much IMO until you get some other stuff to where it will.