Your bicycle looks like it is an American Made brand, Huffy or Columbia maybe? Ross mad some nice one piece crank 27" wheel 10 speeds back in the day. There are 27"x1 3/8" tires available and they may fit in your frame. If you change to 700c wheels then you may be able to fit 35mm wide tires. Check you clearances on the front fork and the rear stays. Sometimes the rear brake bridge is low so check it also. My Varsity in the above pictures has a Profile crank. Profile are BMX/MTB crank manufacturers and their cranks have the option of having bearing cups that fit the old American style one piece bottom bracket. If your seat post is 13/16" like the Schwinn Varsity then Porkchop BMX has aluminum posts available that flair up to 7/8" for the seat clamp. Nice way to drop some weight for little money. I love the older steel 10 speeds with 27"wheels. They have held up so well for decades proving to be a good design. I have come to appreciate the 27"wheel size more now that I've ridden and owned so many different sizes. It is a good all around wheel size for riding both paved and dirt roads. You have done a righteous job of putting on upright handlebars with the correct brake levers and shifters. I run 10 speed cassette hubs on my Bicycles and some have index shifters and one has old friction MTB shifters very similar to yours. I don't prefer one over the other. The frictions are just a tiny bit more fussy but are dead reliable. My Microshift index shifters have been wonderful and reliable but no real preference. I've not heard of friction brifters but someone may have built them long ago. All the brifters I've heard of were for drop bar bicycles. I have an old Nishiki Sebring with stem shifters, drop bars, brakes with the extra levers that allowed you to brake with your hands on the straight flat section of the bars. I'm rebuilding that bicycle at the moment and am going to keep these components. I'm updating the rear end with a cassette hub and a 10 speed cassette run by a Shimano 9speed MTB derailleur. The old stem shifters have enough cable pull to operate a 9 speed derailleur. So the shifting operation will remain a friction setup. Keep up the good work.