I'd bite the bullet and go for the real-deal forged LoPros. Look at Eddyline welding and a couple of the other custom frame shops--they're doing welded/pinned frames, but still using LoPros on the cooler/seat bars and oar towers.
Here's my own frame and what you can do on a budget: 1 1/4" EMT conduit, very close to 1 5/8" OD and will fit LoPros. The oar towers are welded to the outer rails, the inner/outer double rail sections are one piece on each side. Each crossbar is pinned to the inner rail. Chain link top rail sleeves fit close but loosely inside the crossbars. I used Harbor Freight PTO pins to pin the crossbars to the inner sleeve. I sometimes wish I'd have used NRS LoPros on at least the inner two bars (in my footwell).
I used Hollender fittings for the cataraft drop rails. They're 1 3/4" ID, so I used a slit ring of EMT to sleeve the conduit and bring it out to 1 3/4" and fit inside the Hollender.
Oarlock sockets are 1/2" steel pipe, drilled out to 5/8". They're not adjustable, but there are 3 sockets in each tower, so I can move them in 2" increments.
I think I have under $150 in the entire frame. Maybe $40 in conduit that I had an electrician buddy bend the outer rails, oar towers, and footbar. Most of $100 was spent in POR-15 primer and paint.
If you want cheaper LoPros, buy an old used frame.