More on this from Terry Haines:
"Phil,
Do not use the building grade foams on you body main
chassis rails.If you want to use the correct stuff go
to
www.duramix.com ,I think you will find #4330 Pillar
foam will meet your needs as it provides reinforcement
,but use as directed ,this stuff only takes 10 seconds
to rise....Regards TH,Haines Motor Sports Inc."