" I've seen a lot of card caring professionals screw up a lot of stuff because they didn't know what they were doing."
Amen to that statement !
chassis modifications can be done successfully by the "non-professional", provided the "measure 16 times, cut once" principal is followed, and one's welding skills are up to the task so as the modifications are done in a safe manor. by that, i mean the modifications will not compromise the integrity of the chassis, thereby causing safety concerns going down the road.
as to straightening a frame, before a person starts, try to obtain the frame blueprints. these drawings will give you datum points to start your measuring, and will tell you how bad the frame is tweeked. almost all old FSM have these drawings in them. at least the FSM's i have for late 30's, 40's, and 50's have these drawings in them.
if you decide to try to straighten it, remember it will need to be tied down securely, somewhat leveled out as best you can on the non-bent portioned, and slightly over-pushed on the bent section to allow for spring back of the frame material. as to how much over-push is needed, only you can determine that at the time you are doing the push-pulling of the straightening process.
i made a frame table from 6 and 8" I-beam when i built the chassis for my humpback panel truck. i kept the table within a short 1/64" [.015] tolerance to insure the chassis i built would be true and straight.
a person does not have to build a table as i did, but the point is, to make sure the frame is tied down very securely, so as to make absolutely certain your measurements remain constant, and you won't be chasing the frame around due to flexing issues.
i'm just stating my personal experiences, and my opinion.
your mileage will vary.