I use the 273 mechanical rockers myself as I run a flat tappet cam. The stock push rods are too short you’ll need to use after market. Have 894 casting heads with deep dish retainers. 540/560 lift 326 degrees duration. Have no problem with clearance on anything.
Ya, I learned that one the hard way. Well not too hard but some dumb choices to make it work. Anyway I think I have the correct pushrods this time around. Actually, I think stock TA pushrods would work.