A 4.125" bore and 3.75" stroke is a 400, exactly the same dimensions as a chevy 400 smallblock.

First things first, the B and RB have different main bearing diameters. So the crank has to have 2.625" main journals rather than 2.75" 440 journals.

Second, the counterweights need to be trimmed or else the block relieved for the counterweights of the crank. I'd suggest using a cast 440 crank and moving material out of the block.

Third, pistons are going to be expensive if you can't find something close off-brand.

Fourth, I do have a concern about the connecting rod shank running into the lower edge of the bore.

Fifth, everyone and their sister is going to chime in that you should scrap the 361 and build a 383 or 400 stroker.

R.

I just ran some numbers and one possibility is the 3.75 crank, a 6.535" rod, and a Chevy 400 piston. The chevy piston has a 0.927" crankpin. I have a broken set of 6.535" rods with smallblock Shevy big and small ends, so they can be had. Key to this is using off-the-shelf stock pistons.
Break out the calculators!