will 235/75 R15 tires fit on a 98 astro van?

Your original equip was either a 215/75-15 or a 235/65-15.  Using the Discount Tire calculator- http://www.discounttire.com/dtcs/infoTireMath.dos, it appears that the 235/75 is 1.85" bigger around than the 235/65.  I would think that they would fit, but might rub under tight steering situations.  Also, your speedo would be off by over 4 mph at 60.

It's just that I have a set of 4 brand-new snowies left over from the previous full size GM van and would like to keep them. Could I damage the front suspension with a taller tire?

I would sell the larger tires rather than take the risk of having a too-big tire on the vehicle.