The pin can be made from a nail, using one approx. 1/16" in diameter, slightly rounded and polished on each end. Exact diameter is not critical, but alignment is!! The hole MUST be drilled in a press - clamp the new loose forend to a squared piece of 2x4, to assure a perfectly straight hole.

The pin works without a spring since the clamping band is tightened behind it - and the pin only wants to protrude about 1/32" each side.

You might want to locate the hole a tiny bit forward of where it will wind up, that way you can make any final length adjustment with simple sanding under the band before epoxying, rather than messing with the more complicated upper band tenon.

I don't have a rifle handy, but someone will surely give you the offset. IIRC, it is closer to 4" than 3.25", but I could be wrong - wait until you hear from someone who has actually measured it!  Actually, you could probably be off as much as 1/16" or so - bayonets don't fit THAT precisely.