Have you considered using a cheap six-speed? $1800 new from Nissan and plentiful in wrecking yards. See FS6R31A

R31A
2nd 2.324
3rd 1.624
4th 1.271
5th 1.000

60L
1st 3.513
2nd 2.170
3rd 1.378
4th 1.000

Dave the 6 speed does indeed have good ratios, while on the surface modifying the bellhousing seems simple enough actually doing it properly is not. I would expect to have 8-10 hours minimum in it, at $120-150 per hour it would get expensive fast. I've seen the trannies for as little as $600 used, add the $1200-1500 for the machining you are looking at very likely spending $2000. Most of the vintage racers I've seen have custom bellhousings done have spent around $2500.

I've researched the various options for months now and there really isn't a low buck solution. The cheapest seems to be a Ford T-9 out of a Merkur XR4TI / Sierra coupled to an A series automatic bell housing via a flat adapter plate. T9s can be had for as little as $300, the bell set me back $80 and I can actually fabricate most of the adapter plate. Quaife makes a close ratio gear set for $800. There are two companies that make small clutch disc for F2000 race cars that will,fit both the T9 input shift and the A series flywheel. All in the T9 set up may be possible for $1500.

Tom