OK,for clarity ,The male dovetail attached to the Lyman sight I will call the "sight dovetail"
The female dovetail associated with the rifle barrel I will call the "base dovetail"
The sight manufacturer has to decide how to control the tolerances/dimensions on the sights they sell to keep the customers happy. Obviously you don't want a loose fit. The difference between a loose fit and too tight,so you need too big of a hammer ,is very small. .001 in makes a difference. The sight manufacturer typically gives you a little bit of extra steel on the sight for fitting.Its normal to have to fit the sight.If your Lyman 17 and the TC Seneca have the same nominal dovetail size,such as 3/8 or 7/16,then you should not have to remove much steel.
And,in that case,the advice to flat file the underside of the dovetail attached to the Lyman sight is good advice. The angles and slight taper are preserved and all you have to do is keep the flat underside flat
Generally you want to do the filing/fitting on the replaceable part,usually the sight,rather than te gun's dovetail.
There can be exceptions. If you have decided,for example,to fit a sight with a 7/16 dovetail to a rifle with a 3/8 dovetail,recutting the dovetail in the barrel might be practical. I'd still suggest a mill and a dovetail cutter for most folks.
But,once again,if the TC rifle dovetail is cut into a separate sight base that screws to the rifle barrel,I'd get a new sight base from TC. It would be an inexpensive,replacable part. You can fit your sight dovetail to the replaceable dovetail base.,then screw the new base onto the rifle.
That way,if anything goes wrong,all you are out is the price of the TC sight base. Order another. And,if you do it this way,a 3 corner file wth a safe flat side will work to open the dovetail on the replacement sight base for the rifle.
Once again,if we are only talking a few thousandths to fit a sight with the same nominal size dovetail,the suggestion to remove stock from the flat underside of the sight dovetail is good advice