Superheavy tank conversions

Hello all,

So a buddy of mine and I are gearing up for a SG2 game and decided to
build up forces from scratch using conventional vehicle conversions and
modern infantry mixed with some sci-fi models.	We are playing in 15mm.

My question is this,  what is the biggest plastic tank model I can
convert from and stay within the conventional vehicles scaling for SG2. 
I am thinking of converting some T-80s into heavy grav tanks and have
looked at 1/76 and 1/72 scale tanks.  HO would be good too but is very
hard to find.  Thoughts?

Eli