I am one that would paint the frame rattle can black, with the inners and rad support. Paint the firewall white (after fixing the rust) if you can get a matching shade of white. If not, you could also paint it black to the break line where it's blue now...
Rattle can (if prepped right) can work well in situations like this where conditions are not ideal for a high quality automotive paint. And it won't have to be buffed to bring out the shine or get rid of orange peel.....(think about all the bumps on the firewall
)
Since you said you weren't good at painting I'll throw in my 2cents because I've been very successful with rattle can looking very nice....for a long time.....in applications like this....
The trick to rattle can (as any other paint) is proper prep...getting it really clean and ready to accept the paint....and then keeping the spray distance consistent. Try a patch piece of metal first so you don't get runs on your truck.
Find a good distance for you and do light coats. The first coat should not cover all the old color. If it does you're probably too close or keeping the spray in one spot too long. Use overlapping strokes so you don't get lines. Try 2-3 coats. It sounds like a lot of work but would take the same time as one heavy coat and you'd have runs with the one heavy coat. Make sure to follow the directions on the can which usually say you have to recoat within an hour or if not....wait 48 hours....
If you don't follow those directions (within 1 hr or after 48) the paint will look like crackle finish as all of the solvents will not have leached out yet. so try to do the 2-3 light coats within an hour....you can probably do the firewall, inners, and rad support, then back to the firewall and repeat...
Hope that helps
