It just occurred to me that the proposal is a straight trade? Don't do it.
The '70 is a bitsa bike -- bitsa this and bitsa that. That's not necessarily a bad thing unless you have to ask what you need to know about it. But you did ask. This tells me you don't have much depth in loop frames and early T frames, which would be needed to keep a 45-ish-y-o bike on the road, especially this one which is obviously someone's project bike. Even if you did know what the implications of mixing loop and tonti would be in this case, you have no idea of the capabilities and skills of the guy who did the work*, and what he used for donors. In short, to you the bike is worth the sum of its parts.
On the other side of the trade you've got a known quantity in a desirable bike from a good era of guzzi history both aesthetically and mechanically. If I was going to trade that, it would need to be a more straight-across value thing. I don't see equivalent value here.
*neither do it, so this is not meant as a poke at whoever did the work