Re: pontoon tow bar?
tow bars have an enormous amount of force on them so if you can get it in front of a cross member and go clear down to the motor pan and attach it really well there, you at least want it bolted to to several cross members with rear facing braces.
Have you looked into something like this
http://www.iboats.com/Seadog-Transo...05700594--**********.113181529--view_id.39715
Easy to install and it will work fine.
Maybe call Crest and see if they can retro fit one to your boat then it would be designed for your baot and you know it would work.
I have seen 6'' diameter 1/2'' wall alumiunm tow poles that were bent so it woud be pretty hard to design your own and have it work and you could really jack up your boat or hurt someone if your design failed.
If you are dead set on doing this make sure you bolt through at least 2 sets of cross member, just going through the deck and it would rip off the first time you used it, again you are dealing with hundreds and hundreds of pounds of force here.