i would start with the rocker and inner rocker. now you have the perimiter for the floor pan and put the support in last. i'm lot a bodyman but i have done a few using this sequence with good success. put the corner in last.
i would remove the bed and remove the bulkhead to see where contact is being made. probably just trim some sheetmetal or elongate a few holes to get the desired width
nice job. i did the same thing using lexus rx350 seats in my '81 c10. biggest challenge is to get them low enough with the crazy floor pan ups and downs. tilt wheel helps. comfortable seating is high on my list of priorities as well as yours.
i would look for a used squarebody plow truck with a good working plow and move the plow frame over to your truck. not a lot of work. dont forget to add timbrin overload bump stops to replace the stock ones. i have been doing exactly what you intend to do for 17 years with a used $300 meyer...
you should be just as aware of tire age as tread. with age they will lose wet weather traction and flexibility as the temperature drops no matter how good the tread looks. in my mind a 5 year old tire is too old.
if you are not adding zink additive to a flat tappet motor you are simply rolling the dice. is $8 gonna break the bank? i lost a cam for that very reason. not worth the gamble
i used LMC on 3 builds over a period of 14 years and never have i felt that the parts were not usable or inferior. patch panels, interior, and smalls. i either have incredible luck or low standards.
congratulations on owning a 400sbc! treat it like a big block (bigger than a 396) it needs to be properly cooled. use a bb rad and do not run it without a shroud. if rad and stat are up to snuff it should give you no problems. do not run it without a shroud. have been running these engines since...
i think you will have a hard time getting 350 ft/lb torque out of that combo. also what is the gear ratio? the right engine with the wrong gears will fall flat on it's face
there are some good videos on youtube pertaining to engine cleaning. i would look there before deciding on a course of action. no need for a teardown in my humble opinion