454/TH400 install questions

[earl84]

I have done some research on here about the engine perches, and I think what I have figured out is that the current perches I have in my 66 C20 that held a SBC 350 will work for the 454, but will have to be moved to the forward set of holes. Is this correct?
Also, I pulled out the engine and SM420 yesterday, and from what I understand, the SM420 trans crossmember will have to be removed and a different crossmember installed further back. Is this correct?
I will figure out the placement of the crossmember when I have taken delivery of the engine and transmission. Can my SM420 crossmember be modified to work easily or is it just too much hassle? I am currently without a welder, but looking to purchase one before too long.
One last question on tips of how to remove the SM420 crossmember. It looks like I can get to the lower rivets easily enough, but the uppers may be an issue. The cab is still on the truck. Planning on a grinder and serious hammer blows to get the rivets out, and drilling if necessary. Any hints?