And sheet metal is particularly hard. Excess heat can warp the metal, leading to headaches that can usually only be corrected w/ body filler.

What is it that you're trying to do, specifically.... :dunno:
Which model year Integra "front"?
Which model year Civic "rear"?
And also, clarify what you mean by "front"and "rear", please.
Do you mean joining two fenders along a cut edge? [ If so, fiberglassing the two halves together would be a better solution, probably..]

If you can answer these questions, I can help you out to a greater degree. Thanks.

-Dave