As noted, m645 is smaller frame size than the hassy's 6x6. The only way you'll get 'more in the frame' is the fact that m645 comes with wider-angle lenses.
Consider that Hassy bottoms out at 50mm. Yes, there's a 40mm, but you damn well pay for them if you can find them, and I'm not counting the SWD. m645 has 45mm readily available, and 35mm are also not very rare. (I'll leave IQ arguments for later, I'm happy with my 45mm but not so much my 35mm, I could very well have a dud though)
Budget $1-200 for a 45mm, maybe up to $300 for a 35mm, double those prices if you want the newer AF version. There's also the new 28mm that'll cost you a few grand.
Don't bother buying new, you can get mint 2nd-hand AF versions for less than half the new price, the older MF versions for even less.

Also, you can get your Hassy lenses on m645 via a $30 adapter (as well as Pentacon 6's Zeissy goodness and Pentax 67 and others). So you won't need to buy new 'longer' lenses, just get the wider-angles and use the same longer ones via adapter (unless you really want AF, in which case you'll need the AF lenses).